Call forwarding scenario

Hi everyone,
I have a PSTN, GMSC and a MSC configuration. A call is made from PSTN(A number) to MSC(B-number). The B number is diverted to C number which is also in MSC. The network has the PRBT feature. In late call forwarding i am able to hear the Ring back tone of C when called to B. But in early call forwarding case, there is only normal ring. Can anyone clarify this scenario. All the calls from PSTN are routed via GMSC.

thanks, //ramanathan