BT's annual revenue, including payments from rivals for the use of the network, amounts to several hundred million pounds.
The plan will force BT to split its businesses in accounting terms by March 1995 to show it pays the same as its rivals for using the telephone lines.
Oftel will monitor and publish the way BT allocates its costs to ensure charges are fairly calculated.
Ultimately, the watchdog may also introduce a limit on how much BT can charge for using the network.
BT attacked the move as 'a costly increase in regulation at time when government attention is focused on deregulation'.
The company had argued its network should be regulated only when other operators have no option but to use BT's system to complete calls.
A spokesman for BT said: 'All this is cranking up the regulatory scenario. We are seeing more intrusive regulation when what we would like to see is some sign that as competition increases, regulation will be rolled back.'
However Don Cruickshank, the director-general of telecommunications, said that the interests of customers came before those of companies and their shareholders.
He said he must give confidence to those willing to invest in new telecommunications services in the UK.