In previous versions of 3CX the phone system was in charge for multi port gateways (ISDN or FXO) to hunt for the PSTN ports. However, after the firmware of the vendors became more capable, 3CX abandoned the idea of multi trunks per single gateway and instead let the gateway build hunt groups over all available ports. The result is a faster outbound call processing and simplified DID management.
We have prepared a simple document which outlines the steps which need to be taken prior to upgrading to V15 in case your 3CX installation still has PSTN gateways in place which feature more than one SIP trunk per device (e.g. Patton). Read the guide.