It usually happens because the SIP server/PBX isn’t properly handling the transfer method (REFER or re-INVITE), or there’s no proper trunk/routing set up to bridge SIP → PSTN. So when the transfer is triggered, the system just drops the call instead of completing it. I can help you fix it 👍 Quick question: are you using blind transfer (REFER) or attended transfer, and what PBX or provider are you running behind your SIP endpoint?