Distinctive ring and ALERT_INFO, how to clear ALERT_INFO header

I am trying to get our phones to use distinctive ringers and I'm almost there. I want calls that ring at all of our extensions at the same time to use one ringer and all other calls (like ones just to one extension) to ring another way.

To do this I've done the following:
- configured the PolyCom 320 phones and GrandStream 2000 to recognize an ALERT_INFO of "RingAll" and to use a different ringer for that.
- Added "RingAll" to the Alert Info field in the Ring Group that rings everyone. (I'm using FreePBX to config this)

So that worked, and anything that goes through that Ring Group has a different ring that i set for that phone. The problem is that if I pick up one of these calls, and then transfer it to another extension, it still rings that way. If I park the call, then pick it back up from park, then transfer it, it still rings that way, I can't get it to go back to the default

So, I think I need to clear the ALERT_INFO on a call after it's answered by someone, but how do I do that?

Thanks!
Grant