Yes, but the access is significantly reduced compared to general population.
Inmates in disciplinary segregation, the SHU, or the hole at most facilities are limited to one 15-minute phone call per week. That is not a lot, and it requires some planning on both ends to make the most of it. Make sure your number is on his approved call list before he tries to use that one weekly call, because a failed attempt due to a registration issue is a week wasted.
There is one exception that changes the picture entirely. If the reason he is in the hole is that he was caught with a contraband cell phone, phone privileges are typically suspended completely until he is released back into general population. A cell phone infraction is treated as a direct attack on the facility's communication control, and the response is to remove phone access as part of the punishment. In that situation there would be no calls at all until he is out of seg.
If the infraction was something other than a cell phone, the one call per week standard applies. Mail is usually still permitted in the hole, so writing letters and having him write back is the most reliable way to stay connected during this stretch. InmateAid can help you keep letters going out consistently if you want to make sure something is arriving for him even in weeks when the phone call does not connect.