I have GPS on my cell phone through Verizon. I can put in the address and it talks to me through my blue tooth. I can answer the phone if it rings and my GPS will interrupt to give me directions, but the person speaking to me on the phone can’t hear it.
I drove all the way across the USA and never lost signal. It was great! I live in Metro Atlanta and thanks to my phone I have been able to find every place I have wanted to go even when I don’t have an exact address.
Here’s how it works. When activated, a standard cell phone GPS device needs to get the current satellite position/trajectory/etc. directly from a satellite signal. The device initially has no idea where on Earth it is, so calculating geolocation can be a little slow.
Most navigational GPS devices will keep track of their last location. When I turn mine on in the morning, it usually shows me sitting in my driveway for a few minutes while it figures out where I really am. Sometimes it has a hard time locking on in time and will ask me to verify the date, and that I haven’t moved many hundreds of miles since the last use.
My cell phone GPS also has a secondary system called WAAS that helps to tune in the signal. I rarely have an occasion to use it, since the main GPS signal seems to work fine.
One problem with cell phone GPS systems is some don’t have the maps built in – they pull them down from the network. Thus, if you’re in an area with bad coverage, you may suddenly lose GPS functions. I’ve had this happen before when I forgot my GPS and used the functionality in my iPhone.