The most likely explanation is that your son is in the SHU, the Special Housing Unit, which is the federal Bureau of Prisons term for disciplinary or administrative segregation.
When an inmate is placed in the SHU, privileges are significantly restricted across the board. Phone access is typically suspended or severely limited, often reduced to one fifteen-minute call per week. Commissary is restricted. Movement is essentially nonexistent. The fact that video visits are still available while phone calls are not is consistent with how some facilities structure SHU privileges, where certain communication formats are permitted while others are suspended depending on the nature of the placement and the facility's specific protocols.
Mail continues to reach SHU inmates, which is the most reliable way to stay connected right now. Letters, photos, magazines, and books can all still be sent and received even during a SHU placement. InmateAid can get a letter and photos to him at FCI Florence without your home address on the envelope, and having something arrive in that environment means more than it does in general population.
The video visit option, while not the same as a phone call, is worth using. Seeing a familiar face carries real weight for someone in isolation, and if the facility is offering it take advantage of it as often as the schedule allows.
If you want to know specifically why he is in the SHU and how long the placement is expected to last, ask to speak with his counselor when you call the facility. They may not share full details but can often confirm basic status information.