When your cellphone says server unreachable it's usually caused by a server on the cellphone network being down or an error with it.
Your cellphone connects to the cellphone towers and then the signal and calls go through data centers with servers for the phone call to go through.
Sometimes the server goes down and sometimes it's a routing error for the cellphone.
Usually just waiting a few minutes or so will fix the issue.
There's nothing you can do about the phone saying server unreachable other than waiting for awhile.
It's an issue with the cellphone companies hardware or routing issues.
If you do notice the call drop and the phone saying server unreachable you'll usually notice there's no bars on the cellphone signal.