Vodafone's strategy of seeking to boost growth by expanding its presence in emerging markets took a big step forward last night when it announced a R22.5bn ($2.5bn) offer for a controlling stake in Vodacom, South Africa's biggest mobile phone operator.

A deal would see Vodaphone, which has a 50 per cent stake in Vodacom, buy a 15 per cent stake from Telkom (NYSE: TKG - News) , the South African state-controlled telecommunications company that owns the other half of Vodacom.

The bid has the approval of Telkom's board and the South African government, the final arbiter of any deal.

"There's obviously a control premium in there but it doesn't look too expensive," said one well-regarded South African telecoms analyst who could not be named due to company rules. "The question for Vodafone going forward is how they manage all these separate African assets."

Vodafone, the world's biggest mobile operator by revenue, is relying on emerging markets for growth and hopes to use Vodacom as a hub for businesses across Africa. It has stakes in mobile phone operations in Ghana, Kenya and Egypt and would be able to exert control over Vodacom's interests in Lesotho, Mozambique, the Democratic Republic of Congo and Tanzania.

A deal is contingent on Telkom distributing the remaining 35 per cent of its holding to its shareholders. Vodacom's net debt, which stood at R5.2bn at the end of March, would be deducted from the final price.

Vodafone is expected to fund the transaction from its existing debt facilities and cash, allowing it to steer clear of the turmoil in global credit markets.

Months of protracted talks have frustrated Vodacom executives, who have bemoaned the difficulty of being answerable to two equally influential shareholders.

Vodacom reported revenues last year of R48bn, a 17 per cent increase on the previous 12 months. It has an estimated 55 per cent market share of the telecoms market in Africa's biggest economy, but South African mobile penetration is now close to 100 per cent, leading Vodacom and MTN, the Johannesburg-listed group that is the continent's biggest mobile phone operator, to seek opportunities elsewhere on the continent.

The proposed deal will come as a fillip to investors in South Africa, where other deals have been scuppered by lack of debt financing internationally. The potential sale of a controlling stake in one of South Africa's biggest companiesmay ease fears that the country's leaders had grown reluctant to sell prime home-grown assets.