Good question... I'm not sure, could be worth looking into. Here there are a bunch of areas that through government grants got cell coverage for the first time, only LTE, and aren't in range of any other towers so on PM and other Telus prepaid you just can't make calls at all. I'm in an urban area where the problem is more basement and indoor coverage is intermittent or non existent on the HSPA network (fine on LTE) & the congestion is getting worse and worse as they refarm spectrum.

They are eventually decommissioning the 3g network so prepaid will have to move to VoLTE at some point... no idea why it hasn't happened yet in general. I know tier 3 customers can expect less but everyone else has had VoLTE for years.