Hi Divyesh,
Did you try editing the extension or going to Settings/Phones Provisioning? This error is due to older service pack from which the system was updated. To solve this issue you need to take a full back up of your system, then uninstalled, install latest exe from website here https://www.3cx.com/downloads/3CXPhoneSystem12.exe
I would also recommend upgrading to v15 which addressed all those issues.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.