DECT phones problem

[vascop]

I have a Virgin Media phone line with three DECT phones attached. When I pick up the phone to answer, one phone - always the same one - starts to make a rapid loud warbling sound which stops after a minute or so.
Does anyone have any ideas about why this happens? Is it just that this phone is faulty? If I pick up the 'noisy phone' it works normally and obviously there is then no warbling. However I usually pick up one of the other two phones and that's why it's a problem.

[AlBags]

Are they paired properly?
Is there an intercom system switched on in the programming in one or more of them?
Are the others (apart from base) linked only wirelessly?

[vascop]

Thanks Al, I'll make some of the checks you suggest.