Each mission on the 99th should have a banner message repeating every 20-30 mins encouraging players to join our TeamSpeak server (at a minimum) but also can be used to convey certain mission information (maybe AWACS freqs or whatever).

You will need to create 2 triggers.

Trigger 1:
Type: CONTINUOUS ACTION
Condition: (blank)
Action: FLAG INCREASE 99 BY 1

Trigger 2 (1 TRIGGER WITH 2 ACTIONS):
Type:SWITCHED CONDITION
Condition: FLAG 99 = 900
Actions:
-SET FLAG 99 = 0
-MESSAGE TO ALL "Join our TS guys!"

This will display a message to all players every 900 seconds.