What is the dB level in phone hearing & speech volumes? Is there a chart comparing brands?

Phone brand advertise more "clarity" or "speech enhancement" but no information on it. Hearing impaired sites specify amplification in terms of the dB level. Some advertise telephones where the user can select the frequency for which amplification is required. The usual phone brands seem to vary widely in the volume, including those with a feature to increase from low to high. Why isn't the information required? Is there a site showing a comparison?