fon.bmpBT, the giant UK carrier, is on the verge of wrapping up a deal with the Spanish start-up FON, which will let BT customers make cheap calls on FON WiFi hotspots.

Reports about the talks surfaced here (and this story has good details), and a source close to FON tells VentureBeat the deal is near complete.

It is significant, because FON’s routers will give BT Fusion mobile customers a way to share access to their home WiFi networks. Then, in multiple areas of the UK, these customers can make near-free Internet calls, by using those networks. This may mean less revenue as calls leave BT’s network. But perhaps BT feels it doesn’t have any choice, as it continues to get undercut by cheaper competitors.

Under the deal, all new ADSL accounts will be FON access points, we’re told.

FON is backed by Google, Skype, Index Ventures and Sequoia Capital.