Difference in wholesale costs between standalone broadband (SoGEA) and broadband with voice bundled is very small. For example Openreach charges CPs £265 pa for the SoGEA 330/50 FTTP product (sauce) whilst its £291 pa for the data+ voice product (sauce). So a difference of just over 2 quid per month. Which explains why some CPs like BT include voice as standard on their broadband offerings. It makes no sense to take out a third party VOIP service where voice is already included by the broadband provider, especially if its just to receive incoming calls.
BT Business FTTP 330/50 -- Netgear RAX200