Patience is key
Your SR will spend the first five to seven days in a different building before they are housed in their permanent ship (barracks). So if you get an address from the recruiter the day after your recruit shipped out and you immediately send a letter, your recruit is not going to get that letter any quicker and in fact, it most likely will be delayed because again, it winds up sitting in the post office. (more on P-Days)
After recruits arrive, all cell phones are boxed up with their personal belongings and stored locally, to be returned to the recruit the day before graduation. From that moment on, the primary means to communicate with a recruit will be through the mail.