Ok, its working now, this is the error that was shown in the 3CX console:
Registration failed for: nnnnn@xxxxxx; Cause: 500 Server Internal Error; from IP:n.n.n.n:5060
but the real issue was the trunks dropping in the first place. I have two internet providers, I forced the 3CX traffic over to the other provider and the trunks stayed up. so unfortunatly, I don't know the details as to why.