I'm seeing some *very* puzzling behavior from a bunch of our clients.  Basically, the INVITE never makes it to the fone, it looks like the upstream router/modem just swallows it.  The weird thing is, tweaking the username slightly resolves the issue, for a while at least.  And then, it stops again.   Anyone seen anything like this before?  Any ideas?  Am I missing something ridiculously obvious?
Fwiw, I've seen this behavior in setups with *no* SIP ALG, and *no* firewall.  That said, it really *does* look like a firewalling issue, doesnt it?

Here is a typical INVITE that is sent out from Kamailio.  The username is mahesh.test204.  The "fix" is to simply change the name to "mahesh.test205", or "mahesh.test20", or "ahesh.test204", etc.  
Is there something obviously wrong with the INVITE perhaps?  Bad format?



INVITE sip:mahesh.test204@173.101.106.53:5060;rinstance=e2f54e94d29118d7;transport=UDP SIP/2.0.
Record-Route: <sip:74.217.82.147;lr;ftag=X8BN4N8vtXKej;nat=yes>.
Via: SIP/2.0/UDP 74.217.82.147;branch=z9hG4bK8e0c.0ded64d5.0.
Via: SIP/2.0/UDP 74.217.82.232;received=74.217.82.232;rport=5060;branch=z9hG4bK3DtFg45Qetv9F.
Max-Forwards: 69.
From: "7732206484" <sip:2065771035@74.217.82.232>;tag=X8BN4N8vtXKej.
To: <sip:mahesh.test204@a5prodsbc.aptela.com>.
Call-ID: 67108c50-1f0d-122e-17b1-001a647938bc.
CSeq: 382107 INVITE.
Contact: <sip:mod_sofia@74.217.82.232:5060>.
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, UPDATE, INFO, REGISTER, REFER, NOTIFY, PUBLISH, SUBSCRIBE.
Supported: precondition, path, replaces.
Allow-Events: talk, presence, dialog, line-seize, call-info, sla, include-session-description, presence.winfo, message-summary, refer.
Content-Type: application/sdp.
Content-Disposition: session.
Content-Length: 246.
Subject: Local.
Remote-Party-ID: "7732206484" <sip:2065771035@74.217.82.147>;party=calling;id-type=subscriber;screen=yes;privacy=off.
.
v=0.
o=FreeSWITCH 1281422601 1281422602 IN IP4 74.217.82.232.
s=FreeSWITCH.
c=IN IP4 74.217.82.232.
t=0 0.
m=audio 13742 RTP/AVP 0 101 13.
a=rtpmap:0 PCMU/8000.
a=rtpmap:101 telephone-event/8000.
a=fmtp:101 0-16.
a=rtpmap:13 CN/8000.
a=ptime:20.





Mahesh Paolini-Subramanya | CTO | mahesh@aptela.com | 703.386.1500 Ext. 9100
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