Almost certainly nothing serious, and the timing is actually the most likely explanation.
Moving from one housing unit or status into general population involves a lot of logistics happening simultaneously. New cell assignment, orientation to the unit's rules and schedules, getting settled with new bunkmates, learning where everything is and how this particular unit operates. Phone access in the first day or two after a move is not always immediate. He may need to update his approved call list at the new facility, get oriented to the phone system in the new unit, or simply navigate a schedule that is different from what he was used to before the move.
Three calls a day is a consistent pattern, which means the silence stands out more sharply than it would for someone who calls occasionally. That consistency working in your favor means that if something were genuinely wrong, it would be unusual enough to surface through other channels relatively quickly.
If another day passes without contact, call the facility and ask to speak with his case manager. Keep the tone calm and genuinely concerned rather than alarmed or demanding. Explain that he normally calls regularly and that the transition to general population appears to have interrupted that. A case manager who sees a partner reaching out thoughtfully rather than frantically is more likely to check in on him and pass along a message asking him to call or write when he can.
The move to general population is a good development overall. The logistics around communication tend to sort themselves out within a few days. Give it one more day before escalating.