I have a customer that just purchsed 3CX v15.5 on-prem PBX Standard 16SC and they have multiple sites that currently use Vonage Hosted VOIP solution at the offsite locations. They would like to be able to dial 4 digit extensions from 3CX to their offsite locations and have 3CX dial their Vonage IP Phones. They obviously want to do the same in reverse, dial from the Vonage IP Phones to their 3CX system via 4 digit extension numbers. They are using 2000 in their Los Angeles Office, 3000 in thier Canada Office and I configured 1000 in thier 3CX location. How would I either tie these PBX's together for the 4 digit dialing?