How to create forward call optionally based on incoming number?

You can use the Call Control API to forward a call to another number. Assumed that you got the incoming call event via webhook. Parse the notification payload to get the call information from the "activeCalls"
try the code below: (assumed that platform is the platform instance you get from the RingCentral JavaScript SDK)

for (var call of activeCalls){
  if (call.telephonyStatus == "Ringing" && call.direction == "Inbound"){
    console.log("CALLER ID: " + call.from)
    if (call.from == "1111"){
      forwardCall(call, "number to forward the call to")
    }else if (call.from == "2222"){
      disconnect(call)
    }
    break
  }
}

function forwardCall(call, toNumber){
  var endpoint = '/restapi/v1.0/account/accountId/telephony/sessions/'
  endpoint += call.telephonySessionId
  endpoint += '/parties/' + call.partyId + '/forward'
  platform.post(endpoint, {"phoneNumber":toNumber} )
    .then(function(resp){
      console.log(resp)
      var json = resp.json()
      console.log(JSON.stringify(json))
    })
    .catch(function(e){
      console.log(e)
    })
}

function rejectCall(call){
  var endpoint = '/restapi/v1.0/account/accountId/telephony/sessions/'
  endpoint += call.telephonySessionId
  endpoint += '/parties/' + call.partyId + '/reject'
  platform.post(endpoint)
    .then(function(resp){
      console.log(resp)
      var json = resp.json()
      console.log(JSON.stringify(json))
    })
    .catch(function(e){
      console.log(e)
    })
}