A Strange phone call

My telephone rang yesterday evening, just as I`d finished watching a television programme.   I looked at the number on the display and saw that it was my mother.   I answered it as normal, and heard her say "Is that you (and my name)?"  I said "Yes"
"it doesn`t sound like you" she said and then she kept repeating my name ...  I kept saying "Yes it is me"
"what the matter?"   - "nothing" I replied
"who have you got there"   - "no-one, I`m on my own" I answered
"you`ve got someone there who shouldn`t be there"  - "there`s no-one here only me" - I again said
"You`re not answering me" 
"Why won`t you answer me?" - "I am answering you" - I said

All the time, I was answering her, but she kept on and on repeating these questions over and over and she was getting more and more agitated and not stopping to listen or wait for a reply, I got to shouting the answers but still she didn`t seem to be hearing me or taking it in.

Then I heard her saying,
"what shall I do, I`m going to get someone to come round and see you, you`ve got someone there who shouldn`t be there"
"I`ll get the police to come round"
"I`m not going to bed until I know you`re alright"
And I just couldn`t get through to her that nothing was wrong, everything was normal.

I began to think that she was having some kind of fit or turn.  
I wondered what to do.   I daren`t put the phone down, because if she heard it cut off I sensed that she`d be even more alarmed.

Then I thought, the only thing I can try is to try changing the phone I`m using and pick up another of the handsets in the house.   So, I tried that, and it worked!  she heard me on that.   Thank goodness. 

So we then experimented and I got her to ring it again and I answered on the first phone I`d been using and she heard me perfectly.  So, what had been wrong, I`ve no idea.