I have that annoying problem too! So I just called tmobile and after 5-10 mins on the phone with them, they were able to change the delay to 30 seconds before forwarding. Thanks for the heads up.
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They can actually make it even longer than 30 seconds, but offer the 30 second limit because most carriers will bill you for the recvd call after 30 seconds of ringing, whether answered or not.