Landline Rings Intermittently

[kmc4505]

My 83 year old mother lives alone and depends on her land line.   Frequently, someone will call her but the phone does not ring.   The callers hear the phone ringing but it doesn't ring in the house, and eventually goes to voicemail.  The family has learned to call right back, and it usually rings then.  She has a base unit with 2 cordless extensions and an old school land line phone.  None of these ring when the problem occurs.

This has been going on for a few months now, and we can't figure out any rhyme or reason to why the phone rings and when it doesn't. 

Any suggestions?  Thank you.