I have a couple of solutions.
1) The service requires that the CapTel user has a domestic telephone number. This would keep international callers at bay, but it would not allow a CapTel user in another country to call into the US, very easily. IMHO, this is the best solution, since CapTel users cannot currently take their CapTels outside the US anyway. VRS with VCO currently works this way.
2) The service requires you to enter the serial number on your CapTel. This would only allow people with certified hearing loss or people who have paid the hefty price for a CapTel. This would keep international callers at bay, as hey cannot obtain such equipment.
3) Allow only US IP addresses to connect. Block proxies, including Tor, as the issue arises.