If you already have FTTC , with the router located remotely from the master socket and the master socket has no mains outlet nearby is irrelevant, your phone connects to the router , so your phone (if it’s corded) is positioned (or repositioned) close to the router .
If the existing master socket is currently used (it has your phone connected to it )and the router is connected to an extension cable socket , and for some reason you don’t want the corded phone near the router , then it may be possible to use a second ‘pair’ in the extension cable , to wire back from the router location socket to the master so a phone can continue to be used there , but as far as I know , this is not BT or Openreach’s responsibility but yours , so it’s a DIY job or a case you employ a tradesperson to do it , the visit you are getting isn’t to do this work , search voice reinjection for online guidance .
If you don’t want your corded phone connected directly to the router located remotely, but don’t want to reengineer your existing cabling , BT will supply a DV adapter, a mains powered box that your corded phone connects to , you can have this in any room where there is power , so possibly not where the phone is now but positioned away from the router in another room .
Most people can do this simple change , moving a phone cord from the wall socket to the router socket without a home visit .
