RINGMAX Amplified Telephone Ringer - Loud Phone Ringer for Seniors & Hearing Impaired - Home & Office Use - Compatible with Landline Phones (Great for Bedrooms, Offices & Noisy Environments)

You must have a power source to get high volume and a separate power supply needs a wired connection which is a a bother. Alkaline batteries last for years in this unit.. This is my second unit. The previous one lasted well over 20 years in a sheltered outdoor location. Could not be easier to install and use.