Working with a NEC switch.

They send the initial INVITE to asterisk with a phantom NEC number in the From. NEC has told us they send the original caller id information as the From in an UPDATE packet.

From what I have read, only headers in the INVITE are accessible. Is it correct that we cannot retrieve SIP headers from this UPDATE packet (occurs after the INVITE)?
This system has to be chan_sip because blind transfers outside of asterisk work using chan_sip. When we looked at PJSIP blind transfers to switches a couple years ago, we found problems with the Asterisk PJSIP support.

Have a great day!

Dan