Early firmware versions of the Phone Switch 300 had a notorious bug: the FXS ports would create a "loop current" that fried cheap analog phone speakers. This was fixed in firmware version 2.1.4.8. Always check your firmware on arrival.
“To switch your active call to the secondary line, press 300.”
This could appear in an IVR menu for multi-line users (e.g., receptionists). The “phone switch” here is not hardware but a feature code on a digital or VoIP phone. phone switch 300
In many corporate phone systems, you might dial a short code like 300 to activate a switching function. Early firmware versions of the Phone Switch 300