IPhone audio equalizer for phone calls

I have moderate hearing loss. It's frequency selective, peaking in the mid/high range. Does anyone know of settings or an app that will affect the audio of a phone call? My hearing aids are above the ear so they don't help much when I use a phone. It doesn't help just to crank up the volume nearly as much as it does to increase things around the 6kHz band. I've yet to find a cell phone or app that let's me do this.