We want to add a person who is in another state to our phone system as though they were simply in another cubicle here at the office. So, they can dial "302" and get the office assistant's extension just like the rest of us here at the office.
Trixbox v2.8.0.4 Asterisk (Ver. 1.6.0.26)
We do have a VPN already set up that the user uses.
Your remote user will need to register their IP phone to your Asterisk-based system, which means you will need to either allow non-local addresses to register, or provide a VPN or similar tunnel so the IP phone behaves as if it is local. Once either of these are accomplished, you simply create a SIP extension and password on the Asterisk server, configure the phone with those same credentials, and begin making calls.
There are risks and responsibilities associated with trusted remote extensions. Weak SIP passwords are often compromised and used to conduct phishing campaigns, toll fraud, and other malicious activity. It's important to be aware of these risks, mitigate them, and monitor for abuse.
That said, Asterisk is often used for this purpose and it works well. Since you're using Trixbox, which uses a rebranded FreePBX web interface to manage Asterisk, you can use any of the FreePBX documentation to guide you. Here's a discussion on How To Setup a Remote SIP Extension. If you need more assistance, you might try the discussion forum at FreePBX.org.
