This makes little sense , if you had a BT landline service ( no BT broadband ) and Virgin were contacted by you to supply a new broadband only service that makes no difference to your BT service , so why are you plugging your phone into the Virgin router , you didn’t even need to tell Virgin about your BT landline ,
if you did tell them about BT , and Virgin have told you to connect your phone to the Virgin router then , even if you didn’t realise it , Virgin are now responsible for your telephone not BT , unless you were having 2 phone numbers , the original BT one remaining as it was , and an extra number from Virgin , accessed via the Virgin router
‘If you didn’t tell Virgin to port the number from BT , it makes no sense how your BT service was affected.
Did you get a sorry to see you leave communication from BT , if you did , Virgin effectively ceased your BT landline on your behalf , as stated , if your incoming calls don’t come through your Virgin router ( but should ) that’s got nothing to do with BT
