How do I send an SMS from the main company number?

Login your account service web with a super admin user, choose the Phone System tab and select the Auto-receptionist menu and change the IVR general settings by select the extension you want to send/receive text messages to/from the main company number. Authenticate your app with that user to send text messages.

You can always double check if the selected extension has the right to send SMS from the main company number by using this code. E.g.

async function detect_p2p_sms_feature(){
  try{
    var resp = await platform.get("/restapi/v1.0/account/~/extension/~/phone-number")
    var jsonObj = await resp.json()
    for (var record of jsonObj.records){
      if (record.usageType == "MainCompanyNumber"){
        for (feature of record.features){
          if (feature == "SmsSender" || feature == "MmsSender"){
              console.log(`This number ${record.phoneNumber} is a P2P SMS number`)
          }
        }
      }
    }
  }catch(e){
    console.log(e.message)
  }
}

Is that your sandbox account or your production account? What is the main company number? For the code, it is a sample code in Node JS, using the RingCentral JS SDK.

You said that you were able to send a SMS from your individual user number, how did you send it? Via API for via the RC app? Last question, are you a developer?