I have recentely set up a server and installed 3CX IP system. I have installed two computers, three mobile phones and a Cisco SPA 303 IP phone for testing purposes. At present it is only internal, external will come later. I can make and receive internal calls to the computers and mobiles with no issues at all, however I can only call the Cisco SPA 303, I am unable to make a call from it. When i try and dial a extension number from the SPA it gets to the second digit of the three digit number, then beeps and says invalid number.

I have trolled google but cannot find any answer.

Could any cisco or 3CX expert point me to my error.