The USA ring voltage is 66v RMS, as limited by FCC. This is a 54v dc level with superimposed 108v peak-peak ringer at approximately 20 Hz. This is lethal level to any human in a 'low-impedance' state. Low-impedance is the result of skin-breaks, broken lesions, immersion with saline solution etc.
It is clearly not SELV. The user end of the ring voltage is protected by a ring-trip relay limited again by FCC to 100 mA peak current. 100 mA is thought the lower limit of ventricular fibrillation in humans of 18 kG or less----approximately the weight of a 2-year old. Customer safety in 'the-old-days' was the result of the ringer voltage being treated as a hazard and covered. The modern modular-jack is safe probably as a result of the close spacing of tip-ring connectors in the jack. Even with a small finger inserted into the jack virtually all of the current flows from contact to contact and not through the body to another ground point. In my personal opinion----- The ringer voltage should be treated as primary power in any situation where access is upstream of the ring-trip relay. Care must be taken to avoid access to the ringer voltage in any situation other than the modular jack. (the jack fails the standard finger test) There have been some modular jacks marketed with an open-wire construction sometimes called 'fish-hook'. These will tend to puncture and capture a finger and must be avoided. I believe UL-1459 specifically prohibits this construction. All comments obviously personal. Bob
