Random Got SIP response 400 "Bad Request" back from Polycom

I have recently upgraded to Asterisk 1.8.5 on one of my client systems. They have Polycom phones with firmware: 3.17. Since doing so, I randomly get:
chan_sip.c: – Got SIP response 400 “Bad Request” back from
back from one of the phones when we try to transfer to it. It is random, and seems to only be that phone to this point. It happened to more phones until I upgraded from firmware 3.16 to 3.17. Now it is only that one phone. Any ideas would be much appreciated

Bump Experiencing this as well. The debug appears to indicate that two invites are being sent.

So the SIP message flow is as follows:

INVITE sip:101@address SIP/2.0
– Called SIP/101-FluentStream
Retransmitting #1 (NAT) to address:port
INVITE sip:101@address SIP/2.0
<— SIP read from UDP:address:port —>
SIP/2.0 100 Trying
<— SIP read from UDP:address:port —>
SIP/2.0 400 Bad Request

This 400 Bad Request causes the call to go directly to VM. Not sure what’s going on, but appears that it may be Polycom firmware related. You ever get a solution for this?

Looks like one of the Trying responses was lost and the Polycom doesn’t like the retransmitted INVITE?