[USR-TC] Hiper DSP

Hi all We have a tc1000 with this configuration: 1- NMC Card 2- Hiper Access router card 3- 1 E1 DSP ISDN Signaling Max users could connect to it are 29, never we don't have 30 online user together. After 29 online user they get Busy Tone. What do you think about this problem? Regards Shahrooz Sharghi