Lots of reasons, but here are some of the most popular:

  - You're not using hardware flow control. To do so, you MUST 1) use
    the ttyf* devices, not ttyd* or ttym*, and 2) use a "hardware
    handshake" 7-wire cable, which you can buy from SGI but usually
    *not* from a Macintosh house. See the serial(7) manpage and SGI's
    modem "faxable", which you can get from the TAC, for details.

  - The modem is configured funny. Look at the configuration scripts in
    /usr/lib/uucp/fix-* and see if there's one for your modem.

  - /usr/lib/uucp/Permissions is wrong. /usr/lib/uucp/genperm will
    generate Permissions entries for all /usr/lib/uucp/Systems
    entries.

  - IRIX 5.2 had several problems with serial I/O, flow control and PPP
    interoperability.  They are fixed in patch 151 and IRIX 5.3.

  - Some Indys have bad serial port hardware. The problem appears only
    when recent software (IRIX 5.2 + patch 151 or IRIX 5.3) is loaded.
    Call the TAC and give them your serial number.

  - See also these writeups on SLIP and PPP:

    Scott Henry's SLIP and PPP WWW page, at
      http://reality.sgi.com/employees/scotth/dialup_support.html
    Jeff Speegle's SLIP/PPP writeup, at
      ftp://viz.tamu.edu/pub/sgi/software/admin/slip-and-ppp-setup
    Christopher Spry's PPP WWW page, at
      http://sprysgi.sghms.ac.uk/~cspry/make_ppp.html
    "Configuring and Debugging SLIP Connections", Pipeline, Jul/Aug 1995
    "Configuration and Use of PPP", Pipeline, Sep/Oct 1995, and
      corrections on p. 24 of the Jan/Feb 1996 issue