Hi Emilio,
Once we had a similar issue.
Sounds to me that SAP CCtr sends a SIP message (e.g. REFER) to the gateway, which the gateway doesn't understand.
I would try to check the configuration of the SIP gateway or try to play around with the following parameters:
SC => Call Switching => Global Switching Settings => Adv. Settings => Use CD-Controlled Transfer
SC => System Management => Channels => Voice Channel => Transfer => Enable Join Transfer
BR,
Thomas