Phone call hours at most correctional facilities are fairly consistent day to day and do not change frequently without a specific reason. The schedule is set by the facility and posted for inmates, and the expectation on both ends is that it stays predictable.
That said, there are circumstances that can disrupt the normal calling window. A facility-wide lockdown triggered by an incident, a shakedown, a count that runs long, or a staffing shortage can all push phone access back or eliminate it entirely for that day. Those disruptions are not the norm but they happen, and when they do there is typically no advance notice to people on the outside waiting for a call.
Weekend schedules at some facilities do differ slightly from weekday schedules, not dramatically but enough to matter if you are timing when to be available. Some facilities run shorter phone windows on weekends due to reduced staffing, while others operate the same hours seven days a week.
The most reliable way to know exactly what the schedule looks like at your person's specific facility is to call and ask. Request to speak with the unit secretary and ask what the posted phone hours are including any differences between weekday and weekend access. That call takes a few minutes and gives you accurate information directly from the source rather than relying on what your inmate tells you, which may not always reflect the current posted schedule.
Knowing the hours on your end means you can be available and reachable during the windows when calls are actually possible, which reduces missed connections and makes the most of the limited access available.