Intercom needed for blind person for house.

My father is looking for an intercom system for his house. He wants the receiver to be able to be carried around in his pocket so when a person calls to the door the receiver rings and he can talk to them from wherever he is in his house. He is blind and has bad experiences with people calling to the door.
I've been looking around for these on the net but can't find it.

Any help would be appreciated.

There are units that wire into your house phone. You could then use a cordless phone as an intercom handset as well.

Have a look

You are brilliant!! Thanks a million. My dad is delighted.