Have a IsDN-PRI 24 B channels. Was up in service, was watching it take calls but after about 2 minutes, saw it go MBUSY. Shows as near end trouble. Did busy/reset/release board, no change, does same thing. Carrier shows circuit up. Replaced Ds1 board, still acts the same. Cabling all secure. In a trunk group with other circuits which are not having any issues. Any other trouble shooting ideas. thanks