I got the answer from Mitel yesterday.
You answer the call from the 5604 phone and when you want to transfer the call, you do the following:
press “more”
press “contacts”
select “central phonebook”
search for a name or number
press “call”
press "hang up" to transfer the call
Do be aware that the caller on other side can hear everything that you eventually will say, during all the process until you press “call”. The caller will only be placed on hold from the moment that you press “hang up”.
If you don’t want the caller to hear anything in the meantime, then you can choose to mute your microphone.
press “more”
press “Microphone off”
Be aware that you have to do same before the person you are calling answers the phone, or she/he will not be able to hear you.
