Call forwarding from one extension to another extension within my company

What was the problem when you transfer a call to another extension? Any error code or error message?

I have the same problem. there is no error code just no option to enter an extension but just the direct number.

Are you using the "/forward" API? If so, I found out that the API reference is missing the option to specify the extension number. See this working function in Node JS

async function forwardCall(call, toExtensionNumber, toPhoneNumber){
var endpoint = '/restapi/v1.0/account/~/telephony/sessions/[telephonySessionId]/parties/[partiesId]/forward'
  try{
    var resp = await platform.post(endpoint, {
      //phoneNumber:toPhoneNumber,
      extensionNumber : toExtensionNumber // NOT EXTENSION ID!
    })
    var jsonObj = await resp.json()
    console.log(JSON.stringify(jsonObj))
    console.log("FORWARDED")
  }catch(e){
    console.log(e)
  }
}

If you forward to a phone number, that phone number must be online with one of the device (like RingCentral Phone app or a hard phone).