Seems like a lot of effort for an edge case. Now you've added a dependency on some network service you need to access (and from WhatsApp's POV: maintain) before placing outgoing calls. If you use GPS instead, you're dependent on having the location permission (which is not needed to run WhatsApp in general), and you're draining everyone's battery and adding latency to get a sufficiently accurate location lock. Doesn't seem like it would be worth the trouble to me.