BT Group PLC is in early talks to buy TelefA3nica SA's U.K. mobile business, O2, as the two former monopolies move to shuffle their product offerings in a converging U.K. telecommunications market. The transaction, if it goes ahead, would see the return of BT to consumer mobile services more than a decade after the 168-year-old London-based group spun off a then-struggling cellphone operator that was O2's predecessor.