Easiest method is to do VRS
22-02 - Set trunks to VRS if POTS, if SIP/ISDN Set to DID and in your DID Translation table set to 5xx (with xx being your message number)
Upload your message using the basic user1 login, or record using 616, 7, 001
25-03 and 25-04 set to your incoming ring group (1 by default)
What this does is the trunk will play the VRS Greeting, and as no option is selected (or an incorrect option if you don't program anything else) the call will route to incoming ring group 1.
The other option that is available, is 22-14, which will allow you to play a VRS message as a greeting of an incoming ring group, however this will still follow normal programming and make phones ring behind the greeting, so customers may not hear the message, if staff answer.