Hi Christian,
The GET Extension Phone Number List would return the list of phone numbers that are used by a particular extension. The numbers from that list would be mapped to a feature named:
"features": [
"SmsSender",
"MmsSender",
"CallerId"
] Which means you could filter the phone numbers on an extension to identify the numbers which could be used to send SMS. We do not have an API to edit the features on the phone number.
However, you could add numbers to your extension.
Sandbox:
You can use sandbox numbers provided in Developer Portal. For additional numbers, you can add up to 4 numbers and extensions to your Sandbox Online account here: https://service.devtest.ringcentral.com/
Here is how you add numbers in the numbers to Sandbox Online account: Adding a Digital Line (KB 3136)
Production:
You are free to add as many numbers but the service web URL would be:
https://service.ringcentral.com/
Feel free to reach out by submitting a support ticket for additional support.
