You probably will need to check with that ‘dial through’ provider, the ones I’ve seen have alternative access methods like 0800 numbers for customers on IP telephony ( like BT with Digital Voice ) the way telephone traffic is routed on IP is not the same as it was on PSTN and the accessibility BT had to give to the competition , given that BT are not the only game in town and have been for years , it’s likely there is no compulsion to provide that access any longer , but as stated it’s not really a BT ‘problem’ if your relative has to change the way they access some other company’s services, they need to ask that particular provider what they need to do once on DV , or simply try it , if it works , great , if not , then it doesn’t.
