Calls can come any day at any time. Don't try to "guess" when one may come.
The San Antonio area code is 210, but while your trainee is in BMT answer all calls no matter what the area code just to be safe.
If you think you may miss a call, leave a voice message just for your trainee with words of love and encouragement, and directing them who to call next.
Be aware that your trainee's Unit number and zip+4 extension could change once you get the address information in the mail. Don't worry! The PSC (Postal Service Center) is great at getting the mail to the trainees.