Dispatch is the 'Overwatch' intercom voice you will hear around the multi-able City's (City 45, City 18, etc). The Combine use Dispatch for Units (Ground Units) to report what the City's Behavior is like, or what Events are happening. City Scanners can also use Dispatch, reporting to Units.
How to use Dispatch Edit
Overwatch Transhuman Arm (OTA), SeC, CmD, and DvLs can report on Dispatch by using /dispatch (message here). Any Units under the ranks (showed above) can not Dispatch any reports. If any servers have Voice Commands, you can make Dispatch say something from Half-Life 2.
How to RP as Dispatch Edit
RPing as Dispatch is kinda easy. You'll be commanded to say something is requested to. If you see any Crime or Violence, you'll have to report on the intercom, alerting Units.