@Biz88
My apologies, I realise I was giving details about wi-fi calling, so please disregard my previous answer.
As @BandOfBrothers says, it's still early days for Vodafone with 4G calling, but I imagine it will grow once they are confident that they've got it working satisfactorily. It hardly seems any time since 4G data was restricted to PayM contracts, and fetched a higher fee, but now it's included as standard in all phone tariffs.
Of course, one of the advantages of PAYG is that you can try out a network at leisure, to make sure it has coverage that suits you. And there's no doubt that Vodafone does have very good coverage - none of the networks have very good coverage where I live (not enough people!), but I'm always amazed when I travel away by quite how good the coverage seems to be now in much of the country.
