Microwaves are 2.4Ghz (125mm wavelength) and their cabinets are designed to stop exactly that wavelength, mobile phones are 800Mhz to 1900Mhz (about 375mm to 160mm wavelength) and the cabinets are not designed to stop those wavelengths so some signal may leak through.
A cordless phone that works on 2.4Ghz would be a good test though.