Presumably you mean FTTP when you say signed up to BT ‘fibre ’ , with FTTP being separate from copper pair based services line ADSL/VDSL ( FTTC ) unless you asked for your phone number to be migrated from Talk Talk then there is no reason why your TT service is affected , unless you separately contacted them yourself and asked them to cease service assuming your FTTP would be installed without any issues.
As far as getting the TT service restored, its nothing to do with BT , but it’s complex , if you did ask for the phone number to be ported to BT ( or returned to BT if it were originally moved from BT to TT ) then if the FTTP install was never closed ( because it wasn’t installed ) then the TT service should continue, if it were closed in error by Openreach so TT were told to release the number , or you independently asked for your old service to be ceased , getting TT service restored could be tricky as once FTTP is thought to be installed it’s the default method of provision and returning to copper isn’t normally available via standard methods.
Ultimately though if you want TT to restore service, obviously BT cannot provide something on another providers behalf , if TT have ‘ceased’ your services its TT than need to do something about it .
