The South Africa Communications Union (Sacu) said it believed Telkom was about to let go of about 9 000 of its 19 000 employees over the next six months, a claim the company said was untrue.
"We are drawing up the papers so that we can refer the matter. We intend going to court on the issue of the company not consulting with us," said Karriem Abrahams, Sacu general secretary.
Telkom dismissed Sacu's claims as speculation and said it was "not targeting specific numbers of individuals."
The company has said it plans to cut out some layers of management but has given no target numbers. It has said it plans to cut its costs by R1bn ($93m) annually over the next five years.
Separately, the Solidarity union, which represents mainly white workers, said it would file a court petition on Tuesday to prevent the company from sacking its members based on race, claiming Telkom might use race as a criteria for laying off staff.
"It is not lawful that you take that approach," said Johan Kruger, deputy general secretary at Solidarity. "Race should be irrelevant when it comes to retrenching people."
Under South African law on black economic empowerment (BEE) companies must increase black ownership and numbers of black workers, although quotas vary from sector to sector.
Kruger, however, acknowledged that the company had not made any suggestion that race would be a factor in any retrenchment.
"It's not been explicitly been stated, but the fact is that it is there," he told Reuters.
"The reasons being put forward for taking race into account is that Telkom has to reach or maintain a certain BEE rating," he said. "The other reason is that they have a duty to reflect the national racial demographic within their workforce. So that is why they may take race into account and we are saying no, those two are not operational reasons when it comes to retrenching."
Telkom said it would consider qualifications and experience in restructuring and underlined that employment equity - the racial mix of its workforce - was only "one of the four criteria that will be applied".
Telkom shares have gained more than 70% this year after new management at the company, which is majority owned by the government, tightened its belt and posted a profit.