There are two things that can cause that message.
The first and most common is this: You are with AT&T.. There are other GSM carriers out there (T-Mobile for example) and AT&T has agreements with many but not all of them (T-Mobile they have an agreement) but if you connect to a network that AT&T does not have a "Roam" agreement with.. You are restricted to 9-1-1 and that's all (Federal law says the carrier HAS to process a 9-1-1 call, even if you have no minutes on a pre-paid or are subscribed to a non-partner carrier).
The other would be a communications emergency or system overload sufficient to require that message. And though I have seen system overloads.. I've never seen that message because of one.
I would not worry about that message..
NOW: What can you do about it?
In some cases the phone will lock on the strongest GSM carrier even though AT&T has a tower a mear 3db further away.. IN cases like this there is often a menu selection you can access (You will need to call 6-1-1 during a time when you have full service to find out how to get there) that will give YOU control over which network you log into.. You can then choose to lock it on AT&T at least for the time being.
But I can't tell you how to do it. You can get help from 6-1-1
Home was where I park it. but alas the.
2005 Damon Intruder 377 Alas declared a total loss
after a semi "nicked" it. Still have the radios
Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times