Avaya IP Phone Voice Problems

We have an engineer who resides in Mexcio.
Our UK office has an Avaya IP Phone system which allows remote users the facility to  plug an IP phone into their home / office broadban and become and extention of the UK based office.
This in turn allows us to have free call between UK and Mexcio all day long (thus saving £££ in mobile phone / land line bills).

My problem lies with one user, who supplier Telmex has a branded Cable router which will not allow you to change the IP address of the box.
As the UK and Mexico locations reside on the identical ip range of 192.168.1.1/24 range.  when ever we plug an IP phone into the broadband line, it connects fine BUT all voice calls are silent.

I've opened up every port required to allow the firewalls to ignore the open ports.  I've changed my home ADSL ip range to be 192.168.1.1/24 and have the same problem (so I know that it's a firewall routing problem).

Is there a way I can get this working WITHOUT replacing the Telmex Cable box in Mexico.
I have phoned the Telmex supplier but every time we change the IP, all routing in the box fails and will only work again after an factory reset.

I've been told that an Apple Airport Express can manage this BUT am I wrong ?
thegiantsmurfAsked:
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Robert Sutton JrConnect With a Mentor Senior Network ManagerCommented:
Ok, you may have to eventually change the Telmex side to a different type of router so that you can change the internal network to anything other than 192.168.1.0 net. Is there a way to change the Mexico side router to "Gateway" mode? Is this option available? Is it a static Ip on the mexico side or is he/she running via dhcp from their provider? What i'm thinking is that when traffic attempts to traverse from Mexico to UK, the UK side sees the same network and attempts to associate it with your local lan on the UK side. If that address is already assigned on the UK lan, the PBX gets confused. Let us know.
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Robert Sutton JrSenior Network ManagerCommented:
Can you just clarify this a little more.. Are you talking about ring down(incoming calls don't make a ringing sound) is silent or your just can't hear others on the line after a call is established? Let us know.
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thegiantsmurfAuthor Commented:
If I make a call from the uk office to him, everything works fine, it ring and there is clear crisp sound.

If he tries to establish a call, (a) there is no dial tone BUT the system still makes the call , the call receiver will ring, but no sound can be heard on both parties phone.

If I plug the phone into a 192.168.2.1/24 broadband everything works fine (both receiving and making a call)
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Robert Sutton JrSenior Network ManagerCommented:
What are you running between the two locations(UK&Mexico)?  Any chance you can give us a more clear picture in a drawing or explanation? Sounds like a site 2 site vpn issue where you're routing the same net on both ends and somewhere a rule is getting confused but I can't say for sure without more information.
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thegiantsmurfAuthor Commented:
Ok....at the uk office we are running watchguard firewall with an Avaya ip office behind it.

An external ip points to the internal ip of the Avaya kit.

The network address of the Avaya kit is on the 192.168.1.1/24 range

At Mexico, our end user has a cable broadband router with an internal ip of 192.168.1/24 range

When the user in Mexico, or any user on the same ip range of 192.168.1/24 range attempts to connect he has no sound.

If we change the ip to be 192.168.2/24 range then all is fine and dandy
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