If your mum is really desperate to keep her number, then she should consider porting her Post Office number first to a voip provider such as yay.com (even if she has no intention of using their service) and then moving it to BT or possibly Sky.
I say this from experience as my line was with Post office last year (originally BT issued number) and then moved to yay.com and if i enter my yay.com number, only BT and Sky allow me to keep the number - yet when I was with Post office, no ISP would allow me to keep my number. On the downside yay.com will charge you £20 for the number migration (takes about 5 working days) and your broadband might be ceased altogether.
FluidOne FTTPoD 330/30
Edited by baby_frogmella (Sat 13-Oct-18 08:25:39)