common things that work on 2.4Ghz (the same as a DECT phones) are
Wireless networks
Microwaves
Baby monitors
Bluetooth
Radio control models
Wireless video senders.
Cordless keyboards and mice.
And obviously, other DECT phones
Plus a whole lot of other low powered radio devices, it is perfectly possible for any of these the cause interference on the phone, so if you have any off these close by keep an eye on whats turned on when you have a problem, including doing a search for wireless networks when you have the problems to see if there are any different ones.
(or it may of course just be a rubbish phone)