Most of the time, your phone is in RX-only mode to save battery. It listenes for incoming data every millisecond or two, but no more. But as soon as you actively use the network, things change. The basestation must receive your signals as well. This may suddenly and dramatically change the perception of "signal quality". Before it's RX quality only, then it's RX+TX quality.
With incoming or outgoing calls, your phone may switch to another technology. While it may be idling in 4G, large parts of the world rely on 2G for voice calls. As soon as the call starts, the phone switched to 2G and naturally shows 2G reception.
GPS lat/long would work fine, but you won't be able to download turn-by-turn directions or map tiles/vectors from Google.
LTE (and 3G HSPA, such as AT&T, T-mo, 3G/4G/LTE Rogers Canada, LTE Bell Canada, LTE Telus Canada) don't have this behavior. Voice calls are packet switched (so you can do voice and data at the same time).
In both modes data connection is required, so I'm mistaken.