How To Setup An Additional Phone In 6.6.x

To setup a softphone as an Additional Phone:

Under Server > IP Configuration page Allow Nat Port Forwarding must be turned On
Under Server > Access Control > All Networks >SIP must be turned On and User API must be turned On
For more information on the Switchvox Softphone app see this article

Go to Setup > Manage > next to the extension number, click on the blue icon under Actions
Click on Create Phone and assign an unused extension number for the Additional Phone, or you can assign another Phone's extension to converge two existing phones.
For Phone field, select Deskphone or Switchvox Softphone **
Enable the Send email to: field by clicking on NO and set it to YES then, enter the email address of extension user.
Enter any label in the Label field (for example, Cell Phone).

**If you select Softphone for the Phone field, it will create a QR code on the main extension. You can login to the main extension to scan the QR code on your cell phone.

To Add External Number > Enter an external phone number
Prefix: if you require a 9 to dial out for the Outgoing Call Rules, enter a 9 here
External Number: enter the 10 digit phone number (based on your Outgoing Call Rules)
Acknowledge Rapid Transfer:

YES indicates that you want to acknowledge (accept) the call before Switchvox completes the transfer, when you use a Rapid Transfer option from the InCall Menu. Switchvox does not transfer the call until you have answered it and acknowledged it by pressing 1 on your keypad.
NO indicates that Switchvox should complete the rapid transfer as soon as the call is answered.

Label: A label (name) for this phone, for easy reference. It must be unique.
Rapid Transfer Key: select a Rapid Dial transfer Key that is not assigned to another Additional Phone.

The number to press on your phone's keypad to complete this transfer.
Each Phone has a label, and is available as a Rapid Transfer option in the InCall Menu. Rapid Transfer is essentially "speed dial" for your InCall Menu.

Save

For more information on how Additional Phones function see this article