3CX Server is based on the SIP protocol for VoIP calls. Voipjet appears to be solely IAX protocol (suitable for Asterisk for example). The two protocols are not compatible or interchangeable. If you wish to use 3CX, and its the best, you cannot use Voipjet. If Voipjet is absolutely required by you, you must research other PBXs.
Of course I could be in error, in which case someone will step in soon and correct me .
You might have to post a link for me to follow - I did a search for Voipjet on this forum and got only two other threads, both indicating Voipjet was being used on an IAX capable softphone, not 3CX. Sorry.