There is a problem in the communication on the ISDN PRI line. To fix the problem check following please:
- Check if you are using straight or crossover cable (you should use the straight one).
- Check if jumpers on the PRI board are set correctly for the NT/TE mode (JP2 and JP3, see the picture below, you can also find more details in the manual).
- Check the Master/Slave settings in the ISDN parameters menu.
- Check settings of CRC of Layer 1 on PRI interface (see the section Gateway configuration>ISDN parameters: CRC in the PRI config tool).