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Is the extension number itself the extensionID? For example, when I GET info for an account, this is the response:

{
  "schemas": [
    "urn:ietf:params:scim:api:messages:2.0:ListResponse"
  ],
  "totalResults": 1,
  "startIndex": 1,
  "itemsPerPage": 100,
  "Resources": [
    {
      "schemas": [
        "urn:ietf:params:scim:schemas:core:2.0:User"
      ],
      "id": "63548308004",
      "meta": {
        "resourceType": "User",
        "created": "2023-11-15T19:17:30.342Z",
        "lastModified": "2023-11-15T19:17:30.342Z",
        "location": "https://platform.ringcentral.com/scim/v2/Users/63548308004"
      },
      "userName": "test.user15@wellthy.com",
      "name": {
        "formatted": "Test User15",
        "familyName": "User15",
        "givenName": "Test"
      },
      "active": true,
      "emails": [
        {
          "value": "test.user15@wellthy.com",
          "type": "work"
        }
      ],
      "phoneNumbers": [
        {
          "value": "326",
          "type": "other"
        }
      ]
    }
  ]
}

So is "326" the extenstion ID? Because when I try to do a get extension phone number list, it gives a 404 when I enter extensionID as 326:

curl --request GET \
  --url 'https://platform.ringcentral.com/restapi/v1.0/account/~/extension/326/phone-number?page=1&perPage=100'\

{
  "errorCode": "CMN-102",
  "message": "Resource for parameter [extensionId] is not found",
  "errors": [
    {
      "errorCode": "CMN-102",
      "message": "Resource for parameter [extensionId] is not found",
      "parameterName": "extensionId"
    }
  ],
  "parameterName": "extensionId"
}

I also want to make sure that I am not changing the company number for our entire account, just the direct number for the user using the extension.

EDIT: I found the issue, I needed to use the "id" not the extension number. I am able to add a phone line now!

The only other thing I have to figure out is if there is an API to make the phone number type fax only,

And also if there is an API that can make that primary number as the caller ID for fax number in user's profile

That is the thing. Extension id (an identifier) is different than extension number (a label).

Don't specify the type when assign a phone number. For some reason, it causes a error Bad request. I will report this to get it fixed.

Great, thanks! But do you know if there is an API to do the other steps I wanted: make the phone number fax only and also make the primary number as the caller ID for fax number?

For changing the phone number type, it was intended to support via the "type" parameter in the update phone number API. But it seems that it would not work that way. So the team may need to provide a different API to do that.

For changing the Fax caller id, use this API instead. Here is the sample code in Node JS to read an extension phone numbers and assign the fax caller id.

async function read_extension_phonenumber(){
    var endpoint = "/restapi/v1.0/account/~/extension/~/phone-number"
    var params = {
      usageType: ["DirectNumber"]
    }
    var resp = await platform.get(endpoint, params)
    var jsonObj = await resp.json()
    for (var record of jsonObj.records){
        if (record.phoneNumber == '+1209683xxxx'){
          set_extension_fax_caller_id(record.id)
          break
        }
    }
}
async function set_extension_fax_caller_id(phoneId){
    var endpoint = "/restapi/v1.0/account/~/extension/~/caller-id"
    var bodyParams = {
      byFeature: [
        {
          feature: "FaxNumber",
          callerId: {
            type: "PhoneNumber",
            phoneInfo: {
              id: phoneId
            }
          }
        }
      ]
    }
    var resp = await platform.put(endpoint, bodyParams)
    var jsonObj = await resp.json()
    console.log(jsonObj)
}

I don't remember if the extension primary number can be set via API or not. But I will have search for it and update you later.

great, that worked! looking forward to the devs fixing the voicefax type issue.