MTN Group has walked away from talks to buy Telkom SA in a deal that would have created South Africa’s largest mobile phone operator.

MTN told Telkom that it ended the talks on Tuesday after it couldn’t get assurances around exclusivity, a representative for Telkom said in response to Bloomberg questions.

Telkom confirmed that MTN had “terminated discussions” related to the proposed deal in a Sens on Wednesday. This saw Telkom’s share price plunging over 22%, trading at around R34.79 a piece just after 10:00.

The companies were also concerned about getting antitrust approval for any deal, people familiar with the matter said, asking not to be identified because the deliberations were private.

The talks initially stalled following an unsolicited approach from Rain Group Holdings, which proposed selling itself to Telkom, Bloomberg reported last week. The uncertainty caused by this rival proposal also contributed to talks breaking down, the people said.

Representatives for MTN didn’t immediately respond to a request for comment.

Telkom shares closed at R44.76 on Tuesday, valuing Telkom at about R23 billion. But the plunge of the stock on Wednesday morning will see its valuation decline further.

The combination of MTN and Telkom would have overtaken rival Vodacom Group in having the most subscribers in South Africa.

The deal would have likely raised a number of antitrust concerns, given that the number of major mobile networks in the country would have been reduced to three from four, with the vast majority of customers using the top two carriers.