For Yealinks:

To find a Yealink deskphones MAC address, you can flip over the phone and look under the power and ethernet ports

You can also check the phone menus by going to Menu>Status and the MAC address will be the second option

For Cordless phones you can find the MAC address on the back of the base. 

For cordless handsets,  to see the MAC of the cordless base they are paired with, hit the "OK" button and then select "Status", then "Base."

For Polycoms:

Newer Polycom devices have a sticker with a barcode and the MAC address above it on the back of the phone. If this is not present, you can also find the MAC address by going through the menus.

Press the Menu or Home button to access the main menu, then use the arrow keys to navigate to Settings or Status. The exact wording may vary depending on the model and firmware version. Next, navigate to Network, then Ethernet, and the MAC address will be on the top line.