Someone posted this on IRC, not looked at it yet but its looks a useful snippet so want to record it here. Credit to a guy called Funk_ in IRC:
If someone else is having troubles with the callerid showing after finnishing an attended tranfer: the following values need to be set: account.x.cid_source = {yealink_cid_source} = 4 | account.1.cp_source = 0 (in mac.cfg) AND account.X.refresh_remote_id.enable = 1 also in <MAC>.cfg
The last parameter is not in the default cfg file.
If someone else is having troubles with the callerid showing after finnishing an attended tranfer: the following values need to be set: account.x.cid_source = {yealink_cid_source} = 4 | account.1.cp_source = 0 (in mac.cfg) AND account.X.refresh_remote_id.enable = 1 also in <MAC>.cfg
The last parameter is not in the default cfg file.