Terminal is POLYCOM, ISDN 128, MCU is 8620. The MCU can set up connection when it call terminal, but the terminal can not set up connection when it call MCU.
The MCU local management console show the information: connection port is cleared
Change the calling mode from 64K*2 to 128K, then it become OK.
POLYCOM terminal usually use 64K*2 mode, if we need connect it to our device the mode should be changed to 128K.