When I call my mother the landline cuts off after a couple of rings or as we connect.
If I call from my mobile there is no problem.
When I call other landline numbers there is not a problem.
... also, by way of seeking to determine whether the problem lies on your mum's side of the equation or yours, does this only happen when you call your mother's number? When you say there isn't a problem when you call other landline numbers, as a ballpark figure, how many other different landline numbers have you called since you first became aware of this problem ( obviously not precisely, but, say, five, twenty, a hundred, more )? When did you first become aware of this problem, and has anything else changed that might contribute to the issue - i.e. have you or your mum changed providers, or even just changed telephones?
Have you tried calling your mother's phone from any other landline ( as opposed to any other telephone on the same landline ), or are you aware of anyone else ( other family members etc. ) having had the same problem when calling your mother?