Telkom needs to find a "worthy successor" to chief executive Nombulelo "Pinky" Moholi, trade union Solidarity said on Monday.
"Moholi is one of several senior executives who have resigned over the past year and her resignation will contribute to further instability in the company," spokesman Marius Croucamp said in a statement.
He said the company had been plagued by problems, such as a R449m Competition Tribunal fine for "bullying" competitors, a failed bid by Korea Telecoms to buy a 20% stake in its business and weak financial prospects.
"Solidarity is concerned about who will take over from Moholi, as her successor will have to re-establish stability in the company."
Telkom announced Moholi's resignation on Monday.
"Moholi has notified the company of her intention to step down before the end of her contract period," it said in a statement.
"She has given the board the requisite six months' notice in this regard."
The company's board director Neo Phakama Dongwana also resigned.
Once the "current vacancies" on the board were filled, Telkom would start looking for a new chief executive.
reported that at Telkom's annual general meeting (AGM) in October, Communications Minister Dina Pule voted off four board members.
The AGM was chaired by Lazarus Zim who announced his resignation from Telkom in September.
Moholi was appointed chief executive in April last year, after acting head Jeffrey Hedberg decided not to renew his contract.
She was Telkom's chief sales and marketing officer from 2002 to 2005, before moving on to Nedbank for three years.
via I-Net Bridge