I have to assume you are reaching a Ring Central voicemail box. First check the 3CX Activity Log to see if the forwarded cal is even reaching 3CX, or perhaps being rejected. If Ring Central can't complete the forwarded call (timeout?), then the default destination is probably going to be their voicemail.I am forwarding calls from Ring Central as I am waiting on porting. However, the calls are not being routed to Voice Mail in 3CX. If I direct dial my number I get my voice mail. Just not the forwarded calls.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.