cellphone frequencies are much higher than microwave frequencies hence they penetrate through the faraday cage in the microwave. hell i've seen cell phone calls go through a lab faraday cage.
Actaully they are lower.
Most oven operate around 2.4Ghz. (Not only is this convenient for water molecules in food but because there weren't any communications systems at this frequency.
Unfortunately because there weren't any exisitng communications standards using this - it was also chosen for bluetooth and Wifi.)
GSM phones operate at 900/1800 Mhz in Europe or 950/1900 in the US.
The reason your phone rings is that it is very good at picking up extremely weak signals, at least to be able to detect a call. It might not be able to transmit/recv voice while in the oven - try getting inside the oven and answering it.