Don't know about the OP's reasons, but I'm in a similar position of wanting VoIP / 4G Calling but not wanting to leave the O2 network due to network coverage. According to Ofcom figures, EE has the best coverage in our region with 73%, followed by Vodafone 65%, O2 62% and Three 56%, but in my experience O2 has the best coverage, followed by vodafone, then EE (which struggles to provide indoor coverage), then Three (which basically has no coverage anywhere). So Vodafone is the only realistic alternative, but the coverage isn't quite as good as O2, so it would be really nice if O2 would just get their act together.