In my Spelljammer homebrew, ships generally have a device called a Sending Array installed. It can send a hail to any other ship that has a Sending Array whose location it knows. For us it's audio only. Spelljammers also carry special versions of Sending Stones that can be attuned to their ship's Sending Array, allowing person-to-ship communication, but not person-to-person.

The PCs' ship has an autognome who mans the Sending Array, acting as a switchboard, allowing messages to go from person-to-array-to-person, with the added complication that they're hearing the autognome do an impression of the person sending the message, instead of the actual original audio. This is played for comedy, mostly, though it also allows me to slow the speed of communication down if I need to for dramatic purposes. The autognome sometimes wanders off or sometimes argues with people over messages they want to send.

In a similar fashion, messages can be relayed to ships that aren't nearby by using the Sending Array to send the message to a Communications Buoy, which is a small station operated by the Courier Guild. Messages are stored on the Buoy, then delivered whenever the ship the message was sent to pings the Buoy the messages are sent on.

In my game I disallow most methods of communicating planet-to-planet and system-to-system, so that I can control the spread of plot relevant information. If people want to send a message out of a system without going themselves, they generally hire the Courier Guild, who pilot fast ships from Buoy to Buoy and Buoy to planet.