This is a APN issue for sure, the IP5 and IP5s needs to jump back to HSPA+ for phone calls and something is not triggering the switch properly.
I had lunch with my old dev boss, asked him if he's received any calls this week and he said it's been a quiet week, strangely no one has called. So I told him Rogers / Fido has an issue with incoming calls on iOS8 B4, he laughed and thought I was just messing with him. So I called him on speakerphone ... his face turned pale white and he dropped his spoon. Apparently he's been waiting for a very important call, which I didn't find funny at all and felt his pain. He's one of those ppl who doesn't use VM much and had turned off the notifications.