ISPs should absolutely NOT be bungling (typo but I’ll leave it to emphasise the point) a voice service whether it be copper or VoIP with their broadband.
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