A VoiceView-enabled modem mutes the local telephone set when sending data. Muting occurs when the application calls the send or the sendto function or when the application receives notification from a remote modem to receive data. Muting stops when the data transmission is complete.
If an application knows that the remote application will immediately send back data, it can set the VV_REQUESTREPLY flag in the send or sendto function call. This causes the telephone sets to remain muted after the first transmission in preparation for the return transmission. Because the telephone line remains in data mode between transmissions, the time spent switching between voice and data is minimal. Voice communication resumes when all exchanges of data are complete.