For the last 3 months 'the phone' has been calling a specific cellphone number 4-5 times a day.
After speaking with the owner of the number, he has acknowledged receiving the calls and says they sound like a fax machine calling.

The following is attached to the landline:
x3 phones - x2 cordless with built in answering machine (they're a set) and x1 corded.
x1 monitored medic alert

There is a house alarm but it is not monitored.

I have spoken with Spark. They have ruled out faults on the line.
I have spoken to Chubb for monitored medic alert. All ok there.

Spark suggested perhaps it's a setting on the cordless phone that's causing the problem?

Has anyone heard of this happening before or do you have any ideas of what could be causing this?