Call Transfer is used to transfer a call in progress to some other destination. There are two types of call transfer:
While on a conversation with another party, you dial the blind transfer key sequence. MyPBX says "Transfer" then gives you a dial tone, while putting the other party on hold. You dial the transferee number and the caller is put through to that number immediately. Your line drops. The caller ID displayed to the person receiving the transferred call is exactly the same as the caller ID presented to you.
While on conversation with another party, you dial the attended transfer key sequence. MyPBX says "Transfer" then gives you a dial tone, while putting the other party on hold. You dial the transferee number and talk with the transferee to introduce the call, then you can hang up and the other party will be connected with the transferee. In case the transferee does not want to answer the call, he/she simply hangs up and you will be back to your original conversation. Press the disconnect key sequence, set to *0 by default, to return yourself to the original caller.
Note The caller ID presented to the person you are trying to transfer the call to is not what you would expect - Asterisk sets your caller ID to be the extension the call originally arrived at which may not be the same as the extension the call was answered at. There doesn't appear to be any way of getting the correct caller ID.