QuickLinks -- Click here to rapidly navigate through this document

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN ISSUER

Pursuant to Rule 13a-16 or 15d-16 of
the Securities Exchange Act of 1934

For the month of ~~December 2002~~February 2003

SONERA CORPORATION ~~(Translation of registrant's name into English)~~

(Translation of registrant's name into English)

Teollisuuskatu 15
FIN-00510 Helsinki, Finland
(Address of principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover Form 20-F or Form 40-F.

Form 20-F ý Form 40-F o

Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

Yes o No ý

This report on Form 6-K is hereby incorporated by reference in the registration statement on Form F-4 of Telia AB (Registration Statement No. 333-100213).

This report on Form 6-K contains a press release dated ~~December~~February 115, ~~2002~~2003 announcing: ~~MR.~~PRELIMINARY ~~YRJÖ~~RESULTS ~~TUOKKO~~OF ~~CERTIFIED~~TELIASONERA'S ~~ACCOUNTANT,~~MANDATORY ~~TUOKKO TILINTARKASTUS OY, AS A TRUSTEE IN THE~~ REDEMPTION ~~OF THE MINORITY SHARES OF SONERA OYJ~~OFFER.

1(2)

SONERA CORPORATION STOCK EXCHANGE RELEASE ~~December~~February 115, ~~2002,~~2003 at 105.4045 ap.m. PRELIMINARY RESULTS OF TELIASONERA'S MANDATORY REDEMPTION OFFER

TeliaSonera (SSE: TLSN, HEX: TLS1V, NASDAQ: TLSN) today announces that, based on preliminary calculations as of February 5, 2003, holders of shares in Sonera (HEX:SRA), including shares represented by American depositary shares (ADSs), and warrants in Sonera, representing 4.4 percent of the shares and votes in Sonera, after full dilution, have accepted TeliaSonera's MRmandatory redemption offer for all of the remaining shares, including shares represented by ADSs, and warrants in Sonera. ~~YRJÖ~~Following ~~TUOKKO~~the ~~CERTIFIED~~completion ~~ACCOUNTANT~~of the mandatory redemption offer, ~~TUOKKO~~which ~~TILINTARKASTUS~~expired OYon January 31, AS2003, ATeliaSonera's ~~TRUSTEE~~total INholding ~~THE~~of ~~REDEMPTION~~securities OFin ~~THE~~Sonera ~~MINORITY~~represents ~~SHARES~~approximately OF99.4 percent ~~SONERA~~of ~~OYJ~~ the shares and votes in Sonera after full dilution.

~~The District Court of Helsinki has~~Consideration, in at the ~~request~~form of ~~Sonera~~TeliaSonera ~~Oyj~~securities, ~~appointed~~for ~~Mr. Yrjö~~Sonera ~~Tuokko~~securities ~~Certified~~tendered ~~Accountant,~~in ~~Tuokko Tilintarkastus Oy as a trustee (caretaker) to look after~~ the ~~interests~~mandatory ~~of absent shareholders during the~~ redemption ~~of the minority shares in Sonera Oyj under Chapter 14~~offer, ~~Section 19 and Chapter 16, Section 5 of the Finnish Companies Act. The contact information of the trustee (caretaker)~~ is ~~available~~expected atto ~~the~~be ~~Finnish~~delivered ~~Trade~~on ~~Register or at Sonera Corporation~~February 17, ~~Investor Relations, telephone number +358 2040 54254 or +358 2040 58930~~2003.

SONERA CORPORATION

Jyrki Karasvirta, Vice President, Acting Head of Corporate Communications

~~Distribution~~DISTRIBUTION: HEX Major ~~Media~~media

Forward-Looking ~~statements~~Statements

This press release may contain forward-looking statements regarding ~~Telia~~TeliaSonera's ~~exchange~~mandatory redemption offer to Sonera's shareholders and warrantholders. Statements that are not strictly historical statements, including statements about ~~Telia's~~TeliaSoneras and Sonera's beliefs and expectations, constitute forward-looking statements. By their nature, forward-looking statements are subject to risk and uncertainty because they relate to events and depend on circumstances that will occur in the future. ~~Telia~~TeliaSonera and Sonera are under no obligation to, and expressly disclaim such obligation to, update or alter their forward-looking statements, whether as a result of new information, future events or otherwise. Restrictions on Ability of Certain Persons to Participate in the Mandatory Redemption Offer. Due to restrictions under the securities laws of Australia, the Hong Kong Special Administrative Region of the People's Republic of China, Japan and New Zealand, the mandatory redemption offer, insofar as it comprises an offer of TeliaSonera securities, will not be made in any of Australia, the Hong Kong Special Administrative Region of the People's Republic of China, Japan or New Zealand. The mandatory redemption offer will, however, insofar as it comprises an offer of cash, be made in Australia, the Hong Kong Special Administrative Region of the People's Republic of China, Japan and New Zealand. Due to restrictions under the laws of Italy and South Africa, the mandatory redemption offer, both as it comprises an offer of TeliaSonsea securities or cash, will not be made in Italy or South Africa.

Restrictions on Ability of Certain Persons to Participate in the Exchange Offer

Due to restrictions under the securities laws of Australia, the Hong Kong Special Administrative Region of the People's Republic of China, Italy, Japan, New Zealand and South Africa, the exchange offer will not be extended to persons with registered addresses in, and who are residents of, any of Australia, the Hong Kong Special Administrative Region of the People's Republic of China, Italy, Japan, New Zealand or South Africa.

Cautionary Disclaimer/Legend

~~The~~On ~~combination~~December 9, of2002, ~~Sonera and~~ Telia isannounced ~~being~~the ~~implemented~~completion ~~through~~of anits exchange offer ~~being~~for ~~made by Telia to~~ all ~~shareholders~~ of ~~Sonera.~~the ~~The~~outstanding ~~contents~~shares, ofincluding ~~this document are neither an offer to purchase nor a solicitation of an offer to sell~~ shares ~~of Telia. Any offer~~ in the ~~United~~form ~~States will only be made through a prospectus, which is part~~ of ~~a registration~~American ~~statement~~depositary ~~on Form F-4~~shares, ~~which~~or ~~Telia~~ADSs, ~~filed with the U.S. Securities~~ and ~~Exchange~~certain ~~Commission~~warrants ~~(the~~of ~~"SEC")~~Sonera. onEffective ~~October~~December 19, 2002~~. Sonera shareholders who are U.S. persons or are located in the United States are urged to carefully review the registration statement on Form F-4~~, ~~the prospectus included therein and other documents relating to the offer that~~ Telia ~~has~~changed ~~filed~~its orname ~~will file with the SEC because these documents contain important information relating~~ to ~~the offer~~TeliaSonera. ~~You~~As ~~are~~a ~~also~~result ~~urged~~of ~~to read~~ the ~~related~~completion ~~solicitation/recommendation~~of ~~statement on Schedule 14D-9 that was filed by Sonera with~~ the ~~SEC~~exchange onoffer, ~~October 1~~TeliaSonera, pursuant ~~2002~~to ~~regarding~~Finnish ~~the~~law, ~~offer.~~commenced ~~You may obtain~~ a ~~free~~mandatory ~~copy of these documents at the SEC's web site at www.sec.gov. You may also inspect~~

and copy the registration statement on Form F-4, as well as any documents incorporated by reference therein, and the Schedule 14D-9 at the public reference room maintained by the SEC at 450 Fifth Street, NW, Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information about the public reference room. These documents may also be obtained free of charge by contacting Telia AB, Investor Relations, SE-12386 Farsta, Sweden. Attention: External Communications or Investor Relations (phone: +46 8 7137143, or Sonora, Teollisuuskatu 15, P.O. Box 106, FIN-00051 SONERA, Finland. Attention: Investor Relations (phone: +358 20401). YOU SHOULD READ THE PROSPECTUS AND THE SCHEDULE 14D-9 CAREFULLY BEFORE MAKING A DECISION CONCERNING THE OFFER 3

redemption offer to acquire all of the outstanding shares, including shares in the form of ADSs, and warrants of Sonera were not tendered in the exchange offer. The contents of this document are neither an offer to purchase nor a solicitation of an offer to sell shares of TeliaSonera. Any offer in the United States will only be made through a prospectus, as amended or supplemented, which is part of a registration statement on Form F4 which Telia originally filed with the U.S. Securities and Exchange Commission (the "SEC") on October 1, 2002. Sonera shareholders who are U.S. persons or are located in the United States are urged to carefully review the registration statement on Form F-4, as it may be amended from time to time, the prospectus, including any amendments or supplements thereto, included therein and other documents relating to the offer that TeliaSonera has filed or will file with the SEC because these documents contain important information relating to the offer. You may obtain a free copy of these documents at the SEC's web site at www.sec.gov. You may also inspect and copy the registration statement on Form F-4, and any amendment thereto, as well as any documents incorporated by reference therein at the public reference room maintained by the SEC at 450 Fifth Street, NW, Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information about the public reference room. These documents may also be obtained free of charge by contacting TeliaSonera AB, Investor Relations, SE-123 86 Farsta, Sweden. Attention: External Communications or Investor Relations (tel: +46 8 7137143, or Sonera Corporation, Teollisuuskatu 15, P.O. Box 106, FIN-00051 SONERA, Finland. Attention: Investor Relations (tel: +358 20401). YOU SHOULD READ THE PROSPECTUS, AND ANY AMENDMENTS OR SUPPLEMENTS THERETO CAREFULLY BEFORE MAKING A DECISION CONCERNING THE OFFER.

For further information journalists can contact:
TeliaSonera's Press Office, phone +46 8 713 58 30

TeliaSonera, formed through a merger of Telia and Sonera in December 2002, is the telecommunications leader in the Nordic and Baltic regions. TeliaSonera's overall focus is on best serving its customers in its core business and creating value for its shareholders through stronger profits and cash flows. TeliaSonera is listed on Stockholmsbörsen, Helsinki Exchanges, and NASDAQ. Pro forma sales as of December 31, 2001, totalled 80.9 billion SEK (EUR 8.9 billion) and the number of employees as of June 30, 2002, was 30,045.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: December 11, 2002
Date: February 5, 2003
	SONERA CORPORATION
	By:	/s/ JYRKI KARASVIRTA Jyrki Karasvirta Vice President, Acting Head of Corporate Communications
	By:	/s/ MAIRE LAITINEN Maire Laitinen General Counsel

QuickLinks

SONERA CORPORATION STOCK EXCHANGE RELEASE February 5, 2003 at 5.45 p.m. PRELIMINARY RESULTS OF TELIASONERA'S MANDATORY REDEMPTION OFFER
SIGNATURES

Filing Compare · SONERA CORP