No
Is there any way to make that work? Could you plug the phone into a BT router that was bridged to the mikrotik router with the EE 4g sim in it?
The issue we have is that our BT landline is very unstable and down a lot of the time. My parents are elderly and a stable phone line is very important to them for health reasons. I am looking into installing a 4G aerial on the roof for a stable internet connection as we live rurally.
You can buy voip adapters for landline phones that work over most internet connections, but you would need to register with a voip provider. If you needed the voip phone to be on the same number as currently on with BT you would need to port the number to your new provider. If you have Broadband and landline service from BT then porting your number would cease the BB (if you are still in contract you would still be liable for the remaining period). If you only needed to make calls and could manage without porting the number you might want one handset on the BT line and a different one on voip.
Lots of options that might do what you want, but require more details of the objectives.
We have no idea about what you have so can't really answer that directly.
EE are part of the BT Group and will take over the residential side if business eventually.
If you move a landline over to EE there is a significant chance they will move you onto digital voice -BTs proprietary version of voip. They also offer an add on for BB that, iirc, switches to 4g if the normal line goes down - some consider it expensive but whether it is relevant I'm not sure
Thank you for your help, much appreciated.