Alert-Info header unconfigurably passed through to trunk when forwarding

When forwarding a call that is originally tagged with Alert-Info: <>;info=external header via Inbound Routes, the Alert-Info header is passed back out on the trunk.

This presents as a bug to the user where the carrier in question returns 400 Bad Request: Error-Info:<sip:+1xxxxxxx@xxx.xxx.xxx.xxx>;cause="[line 012] SIP syntax error" and call fails.

We are using attached extensions_override_freepbx.conf for now to strip the header on all trunk calls.

Not clear this is a bug or should be stripped by default as interoffice trunks might find this useful but also I see no way a call to the PSTN might need this and no clear way to strip from GUI or easily via dialplan other than overriding large chunks of logic to stop addition of the Alert-Info header.

For what it's worth Set(HASH(_SIPHEADERS,Alert-Info)=) Set(HASH(_SIPHEADERS,Alert-Info)=unset) still result in Alert-Info: being added with the stock dialplan.

Reproducing

1. Set inbound route with Alert-Info: <>;info=external added and routing to IVR.
2. Set extension to forward to external number over outbound trunk route.
3. Dial into IVR and dial forwarded extension.
4. Alert-Info header is sent on outbound route also.

Example

INVITE sip:14130000000@x.x.x.x:5060 SIP/2.0
Via: SIP/2.0/UDP x.x.x.x:5060;rport;branch=z9hG4bKPj0ec89c01-bf5c-411a-91b8-cf127bf22cc8
From: "15080000000" <sip:15080000000@x.x.x.x>;tag=45445379-17b0-4fc5-89bf-74bdf9e4418a
To: <sip:14130000000@x.x.x.x>
Contact: <sip:asterisk@x.x.x.x:5060>
Call-ID: 43b9725d-ad43-46cc-9183-4818b6484aee
CSeq: 18038 INVITE
Allow: OPTIONS, SUBSCRIBE, NOTIFY, PUBLISH, INVITE, ACK, BYE, CANCEL, UPDATE, PRACK, REGISTER, MESSAGE, REFER
Supported: 100rel, timer, replaces, norefersub
Session-Expires: 1800
Min-SE: 90
Alert-Info: <>;info=external
Max-Forwards: 70
User-Agent: PBXact-14.0.13.26(13.29.2)
Content-Type: application/sdp
Content-Length: 235

v=0
o=- 29669853 29669853 IN IP4 x.x.x.x
s=Asterisk
c=IN IP4 x.x.x.x
t=0 0
m=audio 18670 RTP/AVP 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=ptime:20
a=maxptime:150
a=sendrecv

SIP/2.0 100 Trying
Via: SIP/2.0/UDP x.x.x.x:5060;received=x.x.x.x;branch=z9hG4bKPj0ec89c01-bf5c-411a-91b8-cf127bf22cc8;rport=5060
From: "15080000000" <sip:15080000000@x.x.x.x>;tag=45445379-17b0-4fc5-89bf-74bdf9e4418a
To: <sip:14130000000@x.x.x.x>
Call-ID: 43b9725d-ad43-46cc-9183-4818b6484aee
CSeq: 18038 INVITE

SIP/2.0 400 Bad Request
Via: SIP/2.0/UDP x.x.x.x:5060;received=x.x.x.x;branch=z9hG4bKPj0ec89c01-bf5c-411a-91b8-cf127bf22cc8;rport=5060
From: "15080000000" <sip:15080000000@x.x.x.x>;tag=45445379-17b0-4fc5-89bf-74bdf9e4418a
To: <sip:14130000000@x.x.x.x>;tag=SDdvp4d99-
Call-ID: 43b9725d-ad43-46cc-9183-4818b6484aee
CSeq: 18038 INVITE
Error-Info: <sip:+14130000000@x.x.x.x>;cause="[line 012] SIP syntax error"
Content-Length: 0