A more robust solution to this problem is to insure software other than HylaFAX always forces Class 0 when using the modem for data. This however may be problematic with some software packages.
For sharing the line for incoming calls the HylaFAX server must be configured to answer incoming calls using an adaptive answer scheme that distinguishes fax from data calls. For Class 1 modems this can potentially be done by the server. For Class 2 and Class 2.0 modems it is best if the modem supports adaptive-answer in the modem (typically this is enabled with an AT+FAA=1 command). It is also possible for Class 1,2 and 2.0 modems to use the adaptive answer support in the server to answer calls first as data and then as fax; see the descriptions of the ModemSetupAAcmd, AnswerRotary, and AdaptiveAnswer configuration parameters in the hylafax-config(4F) manual page. See also the chapter on Advanced Server Setup.