My experience is limited to a first generation Motorola Android and an iPhone 4. Both had to have cell service to perform as GPS. At one critical point we lost the cell connection along with GPS with the Android.
Though the much greater problem was battery life of that phone. It would not charge and guide at the same time. This would eat up the battery in about 2 hours or less. As the phone aged this got worse.
And when I went to my provider for service, I was told....
You guessed it, "They all do that."
And since that old android was not being supported I got to watch as my apps fell out of date. That lead to the iPhone4.
THis works a little better. Voice directions via bluetooth when I use, Google Maps.
Which was one of the driving reasons for the Android.
Battery life is still an issue. Using just the GPS and cell and shutting down all other apps Strava will suck the battery dry in about 6 hours.
That is providing I don't use the phone for anything else.
That has led me to research handheld GPS / cycling computers and such.
So far the Garmin 810 is in the lead, though I'm not real fired up about dropping $500 for the kit and more for a power meter.
Numbers are fun, but I'm not looking that hard for fun.
I've tied it with MotionX GPS app for iPhone on the motorcycle and car and that can drain the battery pretty quickly as well.
The Garmin 810 is likely much more capable of living life on a bicycle/motorcycle or hiking than my iPhone. As far as I know iPhones don't like water.