I don’t know anything about the PC solution — BUT, you might look into buying a talking caller ID box or phone with one, you can set it loud, so you can hear it in the whole house anywhere, don’t have to be close to a monitor. Not very expensive.
Also, you can get cordless phones that can be turned into speakerphones at the push of a button, some of them are louder than others, you can test them in a store, or buy a quality speakerphone.
Just another option, so your Mother doesn’t have to rush to the computer screen to see who is calling.
One more thing, get an answering machine, that way she can screen to calls and find out who is calling, then decide to pick it up, if it’s someone she wants to talk to.