[VoiceOps] Analog Phone Lines - Timesouce for PBX System

We had a customer that had Frontier Analog phone lines going into their Panasonic TDA-50 PBX system. We recently ported the numbers from Frontier to our Broadsoft, and deployed a Poly OBI508 ATA onsite. We also replaced their router and internet connection. Customer is complaining that their phones are displaying the wrong time. We explained to them that we did not mess with their phone system PBX only the input lines. My question is, do oldschool, onsite PBX systems, like a Panasonic TDA-50, use the analog FXS input lines from the provider (us) as a timing source? If so, what protocol is this, and where would be change it on the adapter? I would think most PBX systems are like computers, and would use NTP as the timing source.