UserAgentPort - unique or duplicated on multiple OBis behind a router?

[CoalMinerRetired]

I've been away from here for a while, and can't find what I remember as an old discussion of the topic.

Should the UserAgentPort, (formally the X_UserAgentPort field under Voice Services > SPx Services) be unique or can it be duplicated on multiple OBi devices sitting behind one router.  In this case a SOHO router, and multiple Obi 202s, plus one Obi110

Note that when one Obi device is configured via the ObiTalk portal, the portal knows to increment the port number, such as 5080, 5081, ...5083 for an Obi202 with all four SPs services configured.

However, when multiple OBi devices are located behind the same router, they all start at the same port no, 5080, 5081, ...5083 and you get two (or three or four) obi devices having the same user agent port.

I seem to recall some discussion on here, that I can no longer find, giving some reasons why you should not duplicate the port 3s across multiple devices, ... it had something to do with the router (or some routers) not cleanly being able to keep track which sip client (NAT-ed host) a SIP packet was destined for.

[Mango]

Many routers will detect multiple devices attempting to use the same port number, and assign a different port number to the second device that registers.  You can go ahead and try it, if you want.  If your router does not have this feature, or it does not work properly, the results should be obvious.

I assign each SP on each device a different X_UserAgentPort, but this is largely a personal preference.

[CoalMinerRetired]

Interesting insights.

To follow up, I read up on the router(s) I have in use, and could not determine if they have this feature. I'm not sure how it would be described or even named.  It's not the usual "SIP ALG" enable/diaable function, right?

So, I went ahead and put the 2nd OBi device's UserAgentPorts on unique ports.  For the record, Google Voice signalling does not use a UserAgentPort (ref), so it's only the true VOiP services that need this.

I use the GV + CC for CNAM arrangement, SP1 + SP2; SP3 + SP4, and became suspicious of this when I noticed some incoming calls would ring (SP4) but immediately upon answering/taking the handset off-hook the call would disconnect. The OBi call history showed the call as ending from my end. This happened irregularly, but often enough that it became suspicious. I haven't had the UserAgent ports changed long to conclude if there is a difference.