Yes, the ring voltage can be adjusted. If you go under INTERFACES and then FXS and go into CONFIG. Under CONFIG you can scroll to the right and the last menu option before TANDEM is the RING VOLTAGE. The default is 50 Vrms, but it can be adjusted up to 70 Vrms.
Hope this helps,
Thanks so much for this information! Initial testing is looking good. You saved me some money and lots of time. Really appreciate it!