Pixel 8 pro drops calls to my Sky home number as soon as it's answered.

[Diire]

My new Pixel 8 Pro works perfectly, apart from dropping any call I make to my Sky Fibre landline. As soon as the call is answered, either in person or answerphone, the call drops - "Call ended" message on the pixel , dead air on the home phone.

NB The Pixel works on all other calls, the home phone works with all other incoming calls. 

Here's what I've tried :

1. Disabled WiFi-calling - Call dropped
2. Updated phone - Call Dropped
3. Pixel + Sky Sim in a Samsung Phone - Call worked
4. Vodaphone Sim in the Pixel - Call worked. 
5. Pixel + Sky sim called another Sky Fibre landland - Call worked.

I don't want to do a factory reset, partly because I don't think that will help - there is something specifically wrong with the combination of my Pixel 8 Pro and my Sky Fibre VOIP service. 

As this is my main ICE contact number,  either my Pixel, my Sky Mobile or my Sky Phone service will have to go if I can't find a fix.  

[Diire]

Update.

6. My Sky SIM + Son's Pixel8 - Call dropped

7. Son's Vodaphone SIM + My Pixel 8P - Call worked. 

The common factor is Pixel 8 and (this) Sky SIM.