Thanks for your reply.
I did try USB power without debugger. It still fails. Which confuses me even more
I also saw the power requirements (2A) and also said to myself "No way will a netduino supply that power". However I have encountered zero issues running directly off the USB cable. It powers up, sends and receives SMSes and phone calls seamlessly. I also have a relay shield which is operating off the same Netduino (trip relays depending on the SMS received) which I'm sure zaps some power too when energizing the relay coil. Everything works perfectly together.
I haven't tried GPRS though... maybe that's where I might encounter power issues
I'm not very experienced in the schematic design etc. I'm a programmer by trade. Electrical engineering isn't my strong point so I won't really delve into cutting traces etc
As a workaround it is very easy to feed a 5V line through a relay and into PIN9 on the shield. And simply use the netduino to trip the relay for 2 seconds. Not clean, but it will do for now...