When I take a call on my handset (Grandstream GXP2000) I have the option to Transfer on 3CX MyPhone (CTI) on my PC but the option for Attended Transfer is greyed out. According to the online manual this should be available in v12.5. This is a really important feature for us as we all handle inbound calls and often need to transfer. Have 3CX and MyPhone v12.5. Why is thi snot available and how do I make it available Help!!