It leaves us all cringing, especially as she can't hear it most of the time so we have to listen for ages before she firstly hears the phone, then locates it in her handbag.
DH and I look across the room with a despairing look, trying to control the bile as it rises.
On our first attempt to raise the issue, I bailed.
DH, 'Mum, I think you need to change your ringtone'
MIL, 'Why?'
Me, 'Yes, DH, why?' 
MIL, 'Its the ringtone it came with'
DH, 'er, um, i don't think you can hear it very well'
MIL, 'yes i can'.
Help MN, wisdom required.