Messenger software that can connect to LAN through internet

[splateffects]

I wanted some help in finding an appropriate software (Preferably open source) which can help sort some communication problems.

Right now in the 2 Branches of my office we use LAN based messenger service (IP Messenger) to chat internally. All our users, except admins/Managers do not have internet access due to data security. The problem is when user in branch 1 wants to send a message to a user in Branch 2. We can't use internet based messenger as users have no internet access. Is their a way this can be resolved by making a server or something in both branches which is on LAN and has internet access too ? 

[Uptivuptiz]

How far apart is the branches? Like room to room or building to building or even further?

[splateffects]

3 minutes ago, Uptivuptiz said:

How far apart is the branches? Like room to room or building to building or even further?

Very Far, Different Cities !

[Uptivuptiz]

1 minute ago, splateffects said:

Very Far, Different Cities !

Oh..... I see the problem. Uhmm I dunno, SMS maybe?  

[dionkoffie]

what if you set up two gateway servers with internet access at both branches. Clients are connected to the server inside the branch and that server passes it to the server from the other branch over the internet. 

[LogicalDrm]

6 minutes ago, dionkoffie said:

what if you set up two gateway servers with internet access at both branches. Clients are connected to the server inside the branch and that server passes it to the server from the other branch over the internet. 

I was thinking something like this, but with private email servers. Users send email to local server. Which forwards it to another server. Which forwards it to recipient. There's probably more elegant way to setup it. With trigger addresses and so on. Like actually having servers share same private database for email. mmy lack of practical knowledge runs short.

[NelizMastr]

If both offices have firewalls (the hardware kind, which you should have tbh) just have a permanent site-to-site tunnel so both LANs can communicate with eachother.

[dionkoffie]

or you could always set up a VPN.