When on a call, click the Transfer button:

The Transfer button will be yellow until you enter the Phone Number or Extension that you would like to transfer the call to. 

Once that information is entered, the Transfer button will be green. Click the transfer button once more and the call will be automatically transferred.

To Transfer using lines:

If you have 2 active calls and need to transfer one to the other, you can do that by using the Single Arrow Transfer button:

In the example above, we have Line 1 as the active call, and Line 2 on hold. 

To transfer, select the Single Arrow Transfer button so that it is highlighted in yellow. Then, select the line you'd like to transfer to. In this example we are transferring to Line 2.

Once you select the line, the call with automatically transfer.