There could be a simple reason, he might have run out of money in his account or minutes available in the month. Most institutions have limits on the number of minutes an inmate can call in a month (not the by the calendar but by date it began). If you are really worried, you can call the institution and have them check on him to be sure.
https://www.inmateaid.com/ask-the-inmate/my-boyfriend-is-at-kirkland-correctional-he-gets-to-make-calls-twice-a-week-and-always-called-me-and-his-sister-he-was-supposed-to-call-yesterday-and-we-did-not-hear-from-him-at-all-now-we-are-both-worried-what-would-be-the-reason-he-wouldnt-get-to-call#answer
Accepted Answer Date Created: March 23,2013