Way off beam, or clutching at straws......

[#1]

....... but just a passing thought here, do routers supplied by either Plusnet or BT have a built in VoIP capability, or will I need to buy an analogue telephone adaptor (ATA). 

I'm guessing they latter, but thought it worth asking you guys.  

[#2]

AFAIK none of the routers supplied by PN have had a voip facility. You will either need an ATA , a Voip DECT system or a router that has a voip facility.

[#3]

Some / many routers with voip don't connect to your phone line. They can't receive calls from the BT phone line and this means you need two phone, one for voip one for the phone line.

The device I use makes calls over voip and also receive incoming calls over my phone line is the Obi110 -- only one hand set (DECT) needed.

https://www.amazon.co.uk/OBi110-British-Adapter-Settings-RealCalls-com/dp/B00K28TJHU/ref=sr_1_3?ie=U...

Unfortunately that device has now reached it end of life and has been replaced by the Obi202, I believe this new device needs an additional USB to phone line adaptor to receive incoming calls from the phone line.