Are your own mobiles with BT? If they are then one option is to contact BT and ask for a Signal Assist unit. It uses your internet connection to generate a 3G signal up to 15 metres. It's better than wifi calling in my opinion.
I'm afraid I don't know what landline calling plans are available with BT, but as nearly all mobile plans have unlimited minutes, I would think that the Signal Assist unit will be the cheaper option.