Straight to voicemail?

Does anyone have a family member that calls you 20 million times a day? I over exaggerate but my mother is that person. She finally got her first Android phone after using a Blackberry for so long. Ever since she made the switch I have not been able to hold a conversation with her on the phone if I try to contact her. Usually, I have to send her a text message and then she calls me. Now I can receive all her incoming calls but she never receives mine. In fact, there's not even a record of me calling her phone under "missed calls". I've put her phone right next to mine and as usual it goes straight to voicemail after one or two rings. Occassionally I get the standard 4 or 5 rings before it hits voicemail but she never picks up the phone when I call. I wonder what the problem is because this doesn't happen with any other call I make.