Just for clarity , dialtone is the noise you hear on a landline phone when you ‘lift the handset’ before you dial the number , the question for people who call your father isn’t if they get dialtone but what tone or message is received, no tone (so silence after the number has been ‘dialled’ , NU tone (number unobtainable) or a voice message like ‘the number you have dialled has no been recognised, please check and try again’ , TBH calling from a mobile is not ideal as they can return nothing on failed calls (therefore not much help ) calling from a landline is more likely to return some sort of tone or announcement, so if possible try a call from a landline , preferably a couple from different providers , as an example) BT/EE and Sky , or Talk Talk , Plusnet , although I appreciate that may not be easy to accomplish.
Presumably your father is now on DV (Digital Voice ) so his phone is connected to his BT router, and the problems with incoming calls began after the change from PSTN to DV , when tbis issue occurs usually it’s either the number has been ported between various providers in the past , or affects just a single mobile operator , if the number has never been ported between various, and always been with BT , all they can do is check the move toDV was successful, if any calls from BT /EE landlines can get through that suggests it was done .
The only example I can think of that’s similar , a BT customer posting on here sometime ago had an issue , some incoming calls didn’t get through , after a little investigation (asking the people who said they couldn’t get through who they used for their outgoing calls ) it was a single company, with a few subsidiary brands , BT had no way to ask them to check their routing information was correct) other networks were fine , in the end that customer had BT renumber their ‘line’ as the other company (if I remember it was his son that was the most important caller who used that company) had asked them to investigate but they were not interested in that one of their customers couldn’t get through to one specific BT number ,
