Q: hearing silence with just one person/#

Hello, my iPhone was purchased in Sept. 2013 and today I discovered that when I call a certain number, I hear silence/the receiver does too. She says it rings at her house (it's a land line).  I have called this person before since I've had this phone.  I checked "do not disturb" and the mute, ringer button on the upper left side.  What could it be?  I did not block her.  How can I double check that I did not?  Did she inadvertently block me (not sure she can with an old fashioned cordless phone)?  She's my mom, and not so computer / iPhone literate.  Maybe she did something?  Or maybe it was me?  I can successfully call and receive calls from other people and she can receive and make phone calls on her side.  This is strange.  Can anyone help before I call my carrier?  Thanks so much.