Head canon time!

Han could have called ahead to Bespin

Sending a signal might give away their position in the middle of space without a hyperdrive, bringing the Empire on them.

Luke could have called The Falcon from Degobah to see if his vision was true

Maybe you have to know where a ship is to contact them. I remember something in Episode II about Obi-Wan having to relay a message to Coruscant through Anakin, but he had to find out that Anakin was on Tatooine beforehand.

The Rebel fleet could have waited safely until the Endor strike team called to say the shield is down

Probably communications jammers all over Endor. And a delay between bringing down the shield and the fleet attacking means the Empire could call in a fleet to defend them (which they had as part of the trap anyway).

Qui-Gon could have called the council/senate as soon as they had cleared the communications blackout on Naboo.

Prequels were not thought out? Alternatively, could be the same as the Falcon, not wanting to call attention to a ship with high priority targets on it with no hyperdrive.

So my end theory is you have to know exactly where the ships are, unless you broadcast and give away your location.

shrug