Library /sys$common/syshlp/nclhelp.hlb set, modem_connect, line, carrier_loss_timer  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: 15000

    Maximum time, in milliseconds, that the carrier detect signal
    can be absent before the loss of carrier event is generated.
    This characteristic is not supported if the characteristic modem
    control is none.