T38 is the standard SIP extension that allow VoIP lines to correctly manage FAX service (both for sending and receiving) using Messagenet VoIP accounts and numbers.
SIP (and VoIP in general) are designed in order to correctly manage voice over IP, and its audio codecs are usually lossy and not suitable for sending and receiving data on resulting audio waves.
This feature is then needed in order to avoid problems and use our VoIP line also for FAX transmissions.
N.B.: this service (FAXes over VoIP Messagenet accounts) is not fully available for every SIP device and is strongly suggested to test this on your SIP platform before implementing it on your production environment.
Sample configuration for T38 on Cisco SPA ATAs
Section "Audio Configuration", page "Line 1/2"
Sample configuration for T38 on Grandstream HT503
Sample configuration for T38 on FreePBX
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