TELECOM Digest OnLine - Sorted: Re: Two VOIP Boxes on the Same Port

Re: Two VOIP Boxes on the Same Port

Gordon Burditt (
Sun, 21 Aug 2005 00:50:23 -0000

> As I've previously mentioned, I've just moved from Columbia, SC to
> Sylva, NC. I took my Vonage phone (Columbia number) with me and got a
> new VOIP phone from Voicepulse (Sylva, NC number). I've got both of
> them connected to my Cisco 831 router via Ethernet cables.

> I've been having some problems getting fast busies when dialing. I
> hang up and dial it a second time and it goes through. Voicepulse was
> not able to sort it out so I contacted Sipura, who was the
> manufacturer of the VOIP adapter (Sipura 3000).

A Sipura 3000 is effectively two SIP devices, one FXS (plug in an
analog phone) and one FXO (plug in a phone line), which can
cross-connect (e.g. dialing 911 can be routed out your local landline,
and incoming calls on the analog line can be routed to the analog
phone). Mine came with the FXS at port 5060 and FXO at 5061. If
yours was preconfigured by a provider, it may be configured
differently, and there may be lots of settings you can't change or
even see at the option of the provider who configured it.

> Sipura said that having two VOIP devices on a single router can be a
> problem. To fix it, they said that you have to change the SIP port on
> one of them. Typically, I believe they said that it was on port 5061
> and 5062. They suggested changing one of them to 5063 or above.

You cannot have two devices on the same public IP and the same port,
as a NAT gateway cannot decide where to send packets coming in from
the outside. Since the Sipura 3000 is two sip devices, you'd need to
set two alternate ports.

If your setup has the two VOIP devices on two different public IP
addresses (no NAT), this should present no problem (except for
bandwidth and latency issues, which mostly has to do with pipe size).

> I called Voicepulse and asked them to make the change. They said that
> wasn't possible. I spoke with the supervisor there. He said that
> their system wouldn't accomodate me using a different SIP port from
> the one I have now.

This may be administratively impractical. If you make an IP-to-IP SIP
call (as your provider will be doing to send a call to you), is there
even a syntax to specify an alternate port? Asterisk does have one,
but it's far from obvious that everything else does. You might be
able to get an alternate port to work for outgoing calls only on one
of the devices.

To take another example, if I set up an *EMAIL* server on an alternate
port, I may be able to specially set up one of my servers to forward
mail there (MX records do not include a port number), but there is no
way to write an email address to get most of the servers in the world
to forward mail there (e.g. doesn't work as an
email address on most servers). I'd have to arrange for the mail to
be routed through a server that uses a conventional port number.

It is possible to set up SRV records to specify what port incoming
calls come in on, if someone is calling using a domain name to dial.
However this does not allow specifying two different ports where the
caller is supposed to intelligently choose which one to used based on
precognition. Providers usually figure out where to send the call
based on registration, which may not track port numbers.

> Can the VOIP experts on here sort this out? To expand upon my system,
> I have a Cisco 831 home/office router connected to Mediacom
> cablemodem. Each VOIP device is plugged directly into an Ethernet
> port on the router.

I presume here that you have *ONE* public IP. This is part of the

> Is Sipura's story plausible? How likely is it that Voicepulse is
> telling the truth about not being able to change the SIP port to
> communicate with their system?

An unlocked Sipura 3000 can be configured for an alternate port on
your end. A locked one might also be configurable as to port number.
The problem is more likely on their end, which does not consider the
port to connect to on an outgoing call to be a variable.

Gordon L. Burditt

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