The basic hurdle in finding a detector is that voltage can be measured passively, in parallel with the line, but current needs to be measured in series with a load, and with the line off-hook.
It is a tariff violation to seize a dial tone without the intention of making a call. The only exceptions are the afore-mentioned VM stutter dial tone detectors. There was a lengthy legal battle to allow that type of circuit to be used in the PSTN. I know one of the expert witnesses who participated in the proceedings on behalf of the manufacturer of one of the detectors.
