Do you configure your TFS to be accessible from outside the network?

• TFS SP1 This feature called "Extranet Support" was added way back in TFS 2005 SP1 as per Stuff in the pipe for Team Foundation Server
• A domain name or IP address forwarded to TFS (eg: tfs.your-domain.com)
• Port 8080 (this is port that TFS uses for source control)
• Firewall/Router rule (ideally)

Tip: You can solve this with TFS Extranet Support:

Yes Port 8080 will work in most cases but not on the strictest networks, where only Port 80 is allowed.
Then you have to use port forwarding via a firewall/router rule (recommended) to forward port 80 to the TFS port, while in this way, you would lose the TFS SharePoint Portal and Reporting Services.

Figure: Rule to forward port 80 to the TFS port