CRC Modem Errors

A frame relay network-via local PTT to X21 Modem to customer router Cisco 3600 s0 interface. The router gets CRC errors. The Keysight Advisor connected between the router and the modem receives FCS errors coming from the PTT line. After connecting the tester in front of the modem G703 we got the errors from the modem. After configuring the modem to C/I bit "1" and TS16 was disabled the cct runs error free.