How to correspond to a Recruit in A School?

They usually have access to computers once they get to A-school. He/she should also be able to have their personal phone when they get to a-school, not to mention access to payphones. The other responses on here are more pertinent to bootcamp, in bootcamp, its true, calls to home are rare and the phones they use in the call center cant be called back. But, A-school is different. They can call everyday if they have a way to do it...however, depending on what branch of the military they joined, the privileges will vary. Navy students get personal phone privileges right away. I met my son just a few weeks ago at his graduation ceremony and brought him his cell phone. Army students for A-school (or AIT as they call it) don't get personal cell phone privileges right away, they usually have to endure the first 2 weeks first.

Write them.. the numbers they call from you can't call back

Letters are the only way. Also, they can call you sometimes, but you can't call them.