Ringtone , what the person making the caller hears , and the called phone ‘ringing’ are separate , so the make and model of the phone itself is immaterial.
With current modern phone systems , effectively an incoming call to your ‘in laws’ doesn’t connect to their ‘line immediately, in a few milliseconds , the ‘exchange processor’ acknowledges the request from whatever ‘exchange’ is making the call ( which could be local , national , international , from BT or any other licensed operator ) and examines the called line , is it free , is it busy , is it out of service ( not paid bill ) , has it got call waiting etc., and returns the appropriate ‘message’ to the caller exchange, in the case where the called line is free , or on a call but has call waiting , the BT processor starts ‘ringing’ the line or applies call waiting signal , and also tells the originating operator exchange to start sending ringtone to their customer on their ‘line’ making the call , it’s only if the call is answered that a connection is made, so for example, if it’s a Sky customer making the call , Sky provide the ringtone , BT ‘ring’ the BT customers line.
In the situation you describe, the BT exchange has done the necessary action ( ringing the line ) and although it’s possible the BT exchange isn’t sending the appropriate response to the originating exchange, because this is a common function rather than individual to one line , the entire exchange would be affected if it were not doing this ,basically everyone on the exchange would experience the same problem, so the ‘suspicion’ would be the ‘exchange’ of the person making the call ….are these calls being originated from other BT customers or other operators ( Sky, Virgin , mobiles , Talk Talk etc ) ?.
Unfortunately, when in comes to interconnection between operators, getting anything done is difficult, if for arguments sake the calls are coming from Sky , or TT or mobiles ( even if it’s a combination of many ) all BT could do is make ‘tests’ from BT equipment to the BT customer, the person making the call that doesn’t receive ‘ ringtone’ would also need to report it to their own provider
