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Thus and Gamma, supported by Cable & Wireless, allege that BT's pricing of wholesale end-to-end voice calls to resellers since April 2005 amounts to an abusive margin squeeze and breaches Chapter II of the Competition Act 1998 and Article 82 of the EC Treaty.
They allege BT's pricing may be below cost, which may force some players out of the market.
"BT has previously been found to have significant market power in a number of network access markets," said Ofcom.
The regulator will investigate the allegations and consider whether BT has abused its dominant position under UK and/or EC competition law.