Welcome to our Community and thanks so much for your first post - I was sorry to understand that your Grandmother is having some issues with getting some incoming calls currently. Can she still call out ok?
Does your Grandmother have any Voicemail or Answering machine linked to the handset that could be stopping the call after just one ring?
What happens when trying to dial your Grandmother's line - does it only ring once from that end also or does it continue ringing? Or perhaps goes to a voicemail/answering machine?
Have you been able to test another handset in the socket at all to see if that helps?
Please do pop back if she's still having some issues and we'll pop you a Private Message in order to take some account information and help you and your Grandmother further.