"Selling a device that does not comply with the regulatory standards is illegal"

Possibly not even selling; in the UK, it used to be [1] illegal to connect a non-approved device to the public network - whether it met the specs or not!

Don't forget: telephone exchanges have some pretty sophisticated diagnostic & monitoring facilities - if you're connecting dodgy equipment, it is quite likely that they will find out!

[1] Not entirely sure what the current UK legal situation is - it's certainly relaxed somewhat with regard to having formal approval.