Switch from BT to VM Landline Voicemail

In December I arranged the migration of broadband and landline (with number porting) from BT to VM for my mother in law who is 83. I registered that she was "Vulnerable" at the time as she has dementia and relies on her home phone for Doctor and family calls. She does have a mobile but the signal is poor and keeps switching the volume down. Its a dementia thing.

The installation went really well and the engineers were great. Tested everything  and the number porting. The calling out was fine from her land line but whenever we call into the number it goes straight onto a message service she has never had. I looked up on the community and found that the problem is down to a service still on the line at BT that needs to be removed by VM directly with BT.

After explaining this to the VM customer service they booked an engineer to come out. He checked all the services and said they are working fine. So I told him to call the number, which he did and it went straight to the voice answer service. I then completely unplugged the landline phone and asked him to ring again. He did and it went onto the answer service again. He said she needed to call BT to get the service removed, to which I said she can't as she is 83 and has no idea what to do. He said he would escalate to his manager. Nothing came back.

I spoken with customer service again a week later and explained the issue. They tested the services again and said its all working. After explaining again that its not they passed me to 2nd line support. I went through everything again and they said they would have to log another ticket to get the engineers to investigate. That was on 27th December and I've still not heard anything. So before I call again, does anyone know if this is actually a porting issue? Is it something that I can press some buttons and get taken off the line or does VM need to get BT to remove an answer service on her line that she never had?