A Microwave Oven should stop any radiation on the same frequency coming out as well as in.
The only mobile Network that uses similar frequencies to it is '3'. So, if the phone were on any of the other Networks that may explain why they will ring inside it.
Are the door seals clean and undamaged?
As another poster commented, the 2.2GHz band is used for everything these days and is like a Sewer of the Airways. Have you tried a different Wireess channel?
You could try a Ferrite Ring (see Maplin) close to where the mains cable enters the Microwave, but if its leaking that much radiation I would stand well clear of it or preferably junk it.
Don't want the old wedding tackle ending up as Microwaved pasta!
Allergy Advice: This post may contain traces of Meerkats.