You need to tell your cisco to use the same link-level protocol as the
other router; by default, ciscos use a rather bare variant of HDLC
(High-level Data Link Control) all link-level protocols use at some
level/layer or another. To make your cisco operate with most other
routers, you need to change the encapsulation from HDLC to PPP on the
relevant interfaces. For instance:

        sewer-cgs#conf t
        
        Enter configuration commands, one per line.
        Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
        interface serial 1
        encapsulation ppp
        ^Z

        sewer-cgs#sh int s 1
        
        Serial 1 is administratively down, line protocol is down 
          Hardware is MCI Serial
          MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
          Encapsulation PPP, loopback not set, keepalive set (10 sec)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
[...]

If you're still having trouble, you might wish to turn on serial interface
debugging:

        sewer-cgs#ter mon
        sewer-cgs#debug serial-interface