Is this a PSTN gateway?
If so...
Is Caller ID subscribed to? Test a line with a stand alone CID device/phone.
Is the correct Caller ID method (for your region/provider) set in the gateway?
If you are using the Bellcore type (North America, and some other regions), is there enough of a wait, after the first ring (CID is sent between the first and second ring), to collect the information?
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.