The company said it has delayed its three-yearly review of its pension funding situation so the trustees and actuaries Watson Wyatt can assess how the guarantee would interact with new regulation.
The guarantee had remained in obscurity but has come under the spotlight in the last few weeks because BT argues it should pay reduced levies to the government's new Pension Protection Fund because of the safety net.
But drawing attention to the guarantee could also encourage private equity bidders - previously deterred by its multibillion pound pension deficit - to come out of the woodwork.
BT said its shortfall had fallen to around £2.5bn at the end of March this year, from £4.7bn in 2004.
The Department of Trade and Industry is in talks with BT over the scope of the guarantee, which would kick in if the phone company were to become insolvent.
Taxpayers are also underwriting other former nationalised industry pensions, including the Railways Pension Scheme.