MyActivity.scala

// in onCreate of your Activity
phone.number.addListener({ myNumberText := phone.number.get() }, executor)

MyPhone.scala

def telephonyManager: TelephonyManager = _
lazy val number: SettableFuture[String] = {
    telephonyManager.getLine1Number match {
      case "" =>
        askUserForHisNumber() // it's empty! Let's ask the user instead...
      case nr =>
        val future: SettableFuture[String] = SettableFuture.create()
        future.set(nr) // That's a bit boilerplate-ish, but since we stick to SettableFuture we have to do this
        future
    }

  }

MyPhoneRest.scala

def askUserForHisNumber(): SettableFuture[String] = sharedPreferencesPhoneNumber match {
    case "" => // so it was not stored in the shared preferences...
      val futureNumber: SettableFuture[String] = SettableFuture.create() // we'll complete it in a bit

      // it's always nice to create an edit text "the way it should work", notice how we add hints and set the InputType to "make sense" in this task
      // I've seen many apps skipping to set the InputType on EditTexts - it's really a pain with these apps... @Devs: just add this one line! :-)
      val input = new EditText(ctx)
      input.setText("+")
      input.setHint("+48555222111")
      input.setSingleLine()
      input.setInputType(InputType.TYPE_CLASS_PHONE)

      val builder = new AlertDialog.Builder(ctx)
      builder.setView(input)
      builder.setPositiveButton(ctx.getString(R.string.done), () => {
        val num = input.getText.toString
        if(num.matches("\\\\+\\d{8,}")) { // just a quick sanity check, if the number makes sense
          futureNumber.set(num) // we complete the future
          sharedPreferencesPhoneNumber = num // that will actually persist the number in the our SharedPreferences, no need for the pref.edit() etc boilerplate when using Scala :-)
        } else {
          askUserForHisNumber() // let's try again ;-)
        }
      })
      builder.setCancelable(false)
      builder.setTitle(ctx.getString(R.string.enter_your_number))
      builder.setMessage(ctx.getString(R.string.please_type_in_your_number))
      builder.show()

      futureNumber
    case num => // yay, it was already stored in shared preferences, let's use it
      info("Phone number was already stored in shared preferences: " + num)
      val future: SettableFuture[String] = SettableFuture.create()
      future.set(num)
      future
  }

normal.scala

val telephonyManager = ctx.getSystemService(Context.TELEPHONY_SERVICE).asInstanceOf[TelephonyManager]
val myNumber = telephonyManager.getLine1Number