Arduino or Raspberri Pi iPhone external ringer

I've been looking on the web for years for an external ringer that could be used for an iPhone or Android phone. They make them for regular land-line phones for the hearing impaired, but unfortunately, not for mobile phones. I have tried countless external speakers, powered, non powered, bluetooth... You name it. The problem is, when a speaker, headphones, etc, is connected to the jack or by bluetooth, the music is loud but when the phone rings, the volume is decreased significantly - I'm guessing so the ringer won't blare thru the headphones? I'm looking for someone that could build something using maybe an arduino or Raspberri Pi or beaglabone, connect via headphone jack or bluetooth, and monitor the phone, If the phone rings, the arduino would set off a 90 decibel piezo buzzer or something really loud. I only want 1 and I'm just an individual with limited funds, However, there has got to be a market for this item. I've searched Google and searched thru countless forums and have found absolutely nothing to fit the bill. Others have also been searching for something like this. The only problem I foresee is the random email at 2am telling you how to save money on your car insurance would also set the alarm off and that wouldn't be good. You'd almost have to go in your phone settings and turn off all sounds but the ringer unless the ardino could be programmed to intercept a particular sound and ignore the other sounds. Thank you for your consideration Donnie H