When someone rings me they don't hear a ringing sound.

Hi everyone,

Well this is bizzare. 

When ever my mum rings my mobile it will ring my end, can answer it etc no problem, but there is no ringing sound on her landline phone when she calls. 

When she rang my old EE number, works perfect, rings another number on smarty down south (I'm in Burnley Lancashire), but when she rings my sister who's on Smarty (lives in Oxford) the ringing sound is there. 

Ive done a full factory reset of my phone (google pixel 6a) still same problem. Drove around 40 miles away, still the same problem.

My mums on BRSK for her landline (fibre to the home) and we've contacted BRSK who have done a full diagnostics of the line with no fault found. 

Anyone else had similar problems like this?

Also it does it wether on 3g 4g or 5g .