Library /sys$common/syshlp/dtehelp.hlb SET, DTE, /MAX_BUFFERS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /MAX_BUFFERS=number-buffers

    Specifies the maximum number of receive buffers.  Receive buffers
    are buffers specially allocated to receive incoming data from the
    modem port.

    The minimum number you can specify is 20. There is no upper limit
    to the number you can specify, but the actual number of receive
    buffers will be limited by your Buffered I/O quota.

    By default, the maximum number of receive buffers is 100.