No phone coverage = no gps.
Sorry, but that is absolute bollocks!! Assisted GPS works very well indeed, without the help of the mobile towers. The towers do however, assist with fast position aquisition after repositioning of the device, and will provide a degraded position while the device is shielded from the satellites, such as in the middle of the CBD etc.
You are right however, to caution against the in-flight use of GPS altitude, as every other aircraft is using a barometrically referenced altitude