My Macs have sometimes lost that connection and needed to have it reestablished. In my case, it was the combination of "Calls on Other Devices" (CoOD) and "Wi-Fi Calling" (WFC) that was lost. (You may not be using Wi-Fi Calling.) Here are some steps you can take to correct the issue:
First, try shutting down both the phone and the Mac, then restarting both. Something that simple might work.
Second, try disabling CoOD/WFC, shutting down both devices, restarting both, then re-enabling CoOD (and WFC if you use it). That should usually work if the first thing didn't work.
Third, if you are using WFC, check your cellular carrier to be sure they know of no technical problem with their local network support of WFC or your account's support of WFC.
Fourth would be to involve Apple Support personnel to see if they have anything to add.