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Appreciate the response Phong! Node is just fine - that is the same function call I used but our params don't quite match up, so I will try it again and report back.

Here is how you can assign an unassigned extension. I assumed that you can read and use Node JS and RingCentral JS SDK. But if not, let me know.

function find_unassigned_extensions(){
    platform.get('/account/~/extension')
        .then(function (resp) {
          var jsonObj = resp.json()
          for (var record of jsonObj.records){
            if (record.type == "User" && record.status == "Unassigned"){
              //console.log(JSON.stringify(record))
              //console.log("======")
              if (record.id == "260981XXX"){ // The extension you want to assi
                assign_this_extension_to_a_new_user(record.id)
              }
            }
          }
        })
        .catch(function(e){
          console.log(e.message)
        });
}

function assign_this_extension_to_a_new_user(extId){
  var params = {
    status: "Enabled",
    extensionNumber: 202, // leave this or make sure the number are not being used
    contact: {
      firstName: "Demo",
      lastName: "Automated",
      company: "Test",
      jobTitle: "consultant",
      email: "valid.email@address.com",
      emailAsLoginName: true
    },
    password: "some-password"
  }
  platform.put('/account/~/extension/' + extId, params)
      .then(function (resp) {
        var jsonObj = resp.json()
        console.log(jsonObj)
      })
      .catch(function(e){
        console.log(e.message)
      });
}