Sounds like it may be a network coverage issue, rather than a problem with the particular phone.
If possible, you need to compare it with another phone on the same network in one of the problem areas.
If both phones give problems, a different network povider may give better coverage in that area.
If only your mum's phone gives problems, the fact that it works in the shop proves nothing. The shop may be in a high signal strength area and the phone would still work with a defect.
It needs to be tested using instruments by the repairers.