[Freeswitch-users] How to get the content of SIP headers and sdp in dialplan?

Well, let me be more precise: In general, I want to be able to modify the SIP headers to my liking. I would like to add a tag to the To-header sent to the B-phone. This tag information can be used at a later stage to put the phone on/off hold from the switch by sending a NOTIFY with an header: Event: hold or Event:talk.