Exactly, so for example if you follow the course that was originally mapped out for you by maps, and GPS doesn't for some reason or another show you on a different road than you're actually driving on like, being on the off ramp when it thinks you're still in the highway, it won't try to recalculate your route. As long as it doesn't attempt to recalculate your route while you're on a call, you're fine. If you manage to get back on the original route the software will recognize that and continue on the originally plotted past. But if you do make a wrong turn and you're on a call, you're going to have to end the call temporarily, let Maps update with a new route, and then you can return the phone call. Again folks, this is only until the update is pushed out which should be in the coming weeks.If Maps has already cached the data needed, and you have the standalone GPS enabled, that should work just fine. But if Maps needs to pull new online data for some reason, it won't be able to if you're on a call of any kind (Bluetooth, speaker, etc) since the phone currently cannot do simultaneous voice and data (except when connected to WiFi).
Generally speaking, Maps does cache all of the necessary information to get you to the destination you've entered at the start of the trip. This is done in order to make navigation seamless and in case you do lose your data connection (and making a call will currently cause you to lose your data connection).