This read-write INTEGER specifies which codec to switch to when fax upspeed occurs, applicable to VoIP applications (with a default of g711u for T1 and g711a for E1); it has no effect for AAL2, which gets its upspeed codec from the profile table instead. Fax transmissions need a higher-fidelity codec than typical compressed voice codecs, so this setting ensures the gateway automatically switches to a suitable codec when it detects a fax tone. For example, if fax transmissions over a VoIP trunk are failing or garbled, an admin would verify vismUpspeedCodec is set to an appropriate high-fidelity codec like g711u/g711a for that line type.