Your ship Comms have a limited range. And comms don't work during Jump. You can't call out or be called. And, faster than light communications DOES NOT EXIST.

At ports, you can send and receive messages. Ships carry those messages to other systems. Therefore, a message to another system takes a week to reach them and then another week for a reply.

I envision it as a network you sync with once you are within range of a network transceiver. You get within range, your ship comms system detects the network, sends any outgoing messages, checks for any new messages, the sync is done. Now, think of what happens next as carrying regular mail. As ships leave ports, bound for different systems, they can take mail as a side job.

So, a ship heading for Zub, for example, takes what is essentially a hard drive with messages bound for Zub. Another ship bound for Forth takes the messages bound for Forth, etc.. So, messages that are sent to systems further than one jump away will make more than one stop and therefore take substantially longer. For example, let's say you are on Nelius and want to send a message to Zub. You type it up, hit send, it goes to the network, then that message will catch the next available ride to the next system over, which is Dalla. From Dalla, it maybe gets carried by a ship heading to Forth. From Forth, it travels to Zub. So, it took three weeks (three jump weeks) to get to its destination. Then, if a reply is sent from Zub, it might get lucky and be carried on a ship headed straight for Dalla, then a ship to Nelius. In this case, a reply would only take two weeks.

Where the system can break down is if you send a message to a system and no ship is immediately headed there. A message could spend several days or longer waiting for someone to be heading that way. On particularly low-traffic ports, this is a common problem.