How to amplify the ringtone volume?

The iPhone has the quietest ringtone volume of all the smartphones I have tried out. Admittedly, I am `hard of hearing´, but many people find the iPhone ringtone impossible to hear if there is ambient noise in the vicinity. The setting is on `loud´ and locked. I use the loudest ringtone. So how can I inclease the volume? I have also tried to connect to a bluetooth lousdpeaker, but this only replicates the original low volume ... even at its loudest setting. I just cannot belkieve that there is no solution to this problem!