Unsure about communications.

[vanir]

Might want to look up the Tales of the Jedi companion and take it at face value.

As described the HoloNet was once a series of navigation beacons. Because they were being used for navigation they weren't used for communications. This was done by subspace transceivers. Problem with subspace transceivers is range and power requirements, they're proportionate. Galactic hubs powered by citywide stations had a sector wide range (hyperwave transceiver) but most worlds had a fairly limited range of realtime communication (subspace transceiver) that was only several systems wide. So Courier-ships were used to transmit messages greater distances, that's why courier ships came into being.

With the development of newer navcomputers and faster hyperdrives (the two are linked) the hyperlane beacons were no longer needed and lay dormant for some time, until Palpatine took the expensive project on to refurbish them and set up the galaxy wide HoloNet using the old hyperlane beacons as the hardware. It is a general transmission medium but his primary reasons was for military purposes. Any vessel with a HoloNet transceiver can communicate with any system connected to the HoloNet in realtime from anywhere in the galaxy, so long as it is within range of any beacon, meaning the major (old republic) hyperlanes.

Essentially if you're in a sector with a major trade route in it, which is most sectors in the galaxy, a HoloNet transceiver will connect, like Vader's ship from the Ison Corridor to the beacons along the Corellian Trade Spine.

Otherwise communications is by subspace or hyperwave and is limited by power supply. Planetary at Stellar Class or better grade gets you sector wide. A backpack with a mini-thermo reactor will do system wide. A light transport can punch out to nearby systems.

Subspace and hyperwave are fine in hyperspace. Not sure about HoloNet, you might have to be in realspace to connect to the beacons, GM rule on that one.
Radio coms, like comlinks won't work in hyperspace to other bodies, fine on board but you need subspace transceiver to talk to other ships travelling in hyperspace with you, being that subspace is the theoretical dimension in Star Trek that runs backwards under realspace. That's why the realtime communications using it across great distances, if you've the power to punch the signal out far. Subspace is sort of like another hyperspace dimension, used for signals in SW rather than travel (other way around in Star Trek). Pretty likely Star Wars starship sensors work in subspace for example, hence their limited range and realtime readings, and you'd need some sort of signal transceiver that works in hyperspace so you can have collision alarms.