When use UC V20, UC V22 IP phone with 3CX PBX, you could use a custom IP phone template to provisioning. The steps are as below.
Create a custom template
Go to "Management Console → Settings → Templates".
Select the htek.ph.xml from the drop-down to make a copy.
Press the Copy button and give it a new name, like ucvxx. Then press OK.
You will get a new template named ucvxx.ph.xml now. Choose all the contents and then delete them.
Then copy the content of the attached file ucvxx.ph.xml and paste to this template.
Press Save button.
Provision extension to the phone
On the extension's Phone Provision page, press Add.
From the drop-down list, choose Htek UCV20-ucvxx (ucvxx.ph.xml).
Fill in the MAC address of the phone and then press Save button.
Select the Provisioning Method according to your needs and then reboot the phone to complete the provsioning.
Auto Provision your Htek UC9 series IP Phone for 3CX v15