It isn't at all difficult to 'spoof' a caller id, and it is something regularly done by the less reputable calling companies.
I suspect your mil's number has been randomly selected by one such.
Frankly, if it is being widely used, (you could Google the number, or check one of the 'whocallsme' sites to get an idea), the the only remedy is to change the number. Any decent telecoms provider should do this on request, particularly if a Google search provides evidence of misuse
Inconvenient, I know, but the best answer.
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Landline hacked? 
Landline hacked? 