Expanding on what mixig has replied with - it does work normally out of the box so to speak (and with another little trick further down.)
Test performed:
EXT 540 = Softphone
EXT 550 = Yealink internal Extension (should behave the same if external call.)
0411111111 = Mobile phone
Call EXT550 from EXT540 = answered - call in progress.
Press Transfer Button on EXT540 Softphone - dial number (either full number, Speed-dial or internal extension) you wish to transfer EXT 550 call to, so 0411111111 in this case...it starts ringing the Mobile - you will get ringing sound from EXT540 and EXT550 will go to 3CX ON-HOLD music during the process.
The Transfer from softphone is not Complete until the Mobile answers, at which point you don't need to do anything, as the moment you answer the Mobile 3CX treats it acts as a Blind transfer and just swaps the connection over. However if you don't answer the call on the mobile and hang up the Softphone, or try to do anything else, it will just drop the call(s)
Effectively it means you can't consider the Softphone to be a Semi-Attended transfer device like a standard VOIP Phone, and should probably not be used in a Receptionist capacity - it should only be supplement to Myphone and proper handset and Expansion boards.
You can get around the Blind transfer issue however by using the same functionality as required when Parking calls on Grandstream phones. (due to the fact grandstream cannot transfer TO "SP" based Shared Park lines as they do not like LETTERS in a dial sequence.)
Anyway - Answer is:
Answer the call on EXT540 (softphone) from EXT550 (yealink) - it will show as Line 1 in softphone. Then press Line 2 - it will put the first call on hold. You then dial the extension/Mobile/External number - it will connect you, at this point you can discuss with the remote party and find out if they want to take a call, and if they do, just press Transfer, then press Line 1 again and the call will disappear from the Softphone and the calls that were on Line 1 and Line 2 will be joined together in a pseudo Conference.
Hope this helps you and others.
Regards
Leigh