Send an SMS to one of your contacts

I have an application and in the end I wanted to send a msg to one of the user's contacts about my application.

var columnNumber = cursor.GetColumnIndex(ContactsContract.CommonDataKinds.Phone.Number);
            contacto_numero = cursor.GetString(columnNumber);         
            SmsManager.Default.SendTextMessage(contacto_numero, null,msg_texto, null, null);

This is my code, and I do not understand why not to send msg , I think I have everything in order.

My msg is like this and has no more than 160 characters.

msg_texto= "O seu amigo " + user_nome + " recomendoou a vir fazer o nosso questionario, venha experirmentar a nossa aplicação. Procure-nos na Playstore";

Thank you so much to anyone who can help