Since I'm green in SIP. I use over two day for study how to set up pbxnsip. However, I cannot make two extensions for internal communication. No need to say for setup a gateway for PSTN call. The remote support for it needs US100 per hour. Then we give up to use it. After then I source the other one. Lucky, we found 3CX. It also need around fifteen mins to config two extensions for internal communication at initial stage. I believed that pbxnsip should be much more mature than 3CX. But 3CX can be very easy to setup your SIP PBX with in an hour.