This one has been answered a few times already on the forum. I'll take another crack at it.
Unfortunately, Cisco IOS H.323 based gateways talking with CallManager with current software will not be able to play a ringback tone during a transfer of an active call, even if CallManager tells it to do so. The IOS folks are working on implementing a fix for this, since CallManager is now able to tell the gateway to play a tone to the PSTN side, but I do not have word on when the fix will be available.
The long term solution is for both IOS and CM to implement the H.450 Call Transfer feature, but that will be a little further away.
Note: this does not happen with Skinny (and I believe MGCP) gateways.