Here is a link to a Grandstream setup, maybe you can mimic.
https://oneview.mitel.com/s/article/Grandstream-HT503Use the following instructions to configure a Grandstream HT813 or HT503 ATA device.
In the address bar of any web browser, type the IP address of the device and then press Enter.
The admin login page is displayed.
Log in using admin as the password.
Select the FXS-Port tab, and configure the settings as follows:
Account Active: Yes
Primary SIP Server:
US/Canada: 3psbc.sky.shoretel.com
UK/Europe: 3psbc.sky.shoretel.eu
Australia: 3psbc.sky.shoretel.com.au
Outbound Proxy:
US/Canada: 3psbc.sky.shoretel.com
UK/Europe: 3psbc.sky.shoretel.eu
Australia: 3psbc.sky.shoretel.com.au
SIP Transport: TLS
SIP User ID:
US/Canada: Enter 1 + the 10-digit phone number of the connected phone
UK: Enter 44 + the 10-digit phone number of the connected phone
Australia: Enter 61 + the 9-digit phone number of the connected phone
Authenticate ID:
US/Canada: Re-enter 1 + the 10-digit phone number of the connected phone
UK: Re-enter 44 + the 10-digit phone number of the connected phone
Australia: Re-enter 61 + the 9-digit phone number of the connected phone
Authenticate Password: Enter the user's voicemail password.[i][/i][u][/u]
Name: Enter the display name for the phone
iLBC Frame Size: 30ms