Telco Global Communications has submitted a complaint to watchdog Oftel, claiming BT is preventing customers from changing suppliers.
The company submitted its official complaint after receiving protests from customers who had had their new services cancelled by BT without their consent.
Under CPS (the carrier pre-selection service), which was set up by Oftel to promote fair competition, consumers should be able to easily switch supplier without changing their BT line or needing to dial prefix numbers.
"As a result of the carrier pre-selection service, consumers can now benefit form cheaper phone calls easily and quickly. However, through a dirty tricks campaign BT is attempting to stop this happening," said Dr Engin Mumcuoglu, managing director at TGC.
TGC claims its customers have been falsely informed by BT that CPS may cause problems with broadband and that it is difficult to leave an alternative provider once selected.
"We have numerous examples of BT either cancelling our customers' orders without their knowledge or not even accepting initial requests for their service to be transferred. We have raised the matter with Oftel and look forward to a speedy and satisfactory resolution," added Mumcuoglu.
BT denies the allegations, but refused to comment further until the investigation has been carried out.