According to page 14 of the ZTE WF723CA manual, which I found online, a 3-way conference call is completed by:
1 - call the first party. After getting connected,
2 - press the [flash] button on your phone to receive a new dial tone and call the 2nd party. After getting connected,
3 - press the  key, then the [flash] key to complete the 3-way connection.
This worked for me a grand total of 2 times, and never worked again! I have cycled power on the device, tried having only one phone connected, all to no avail.
Does anyone have any insight?