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My mom is near-deaf, and has tried a few apps and services like CapTel and ClearCaptions, but has ultimately abandoned them all. For the longest time, whenever she got a phone call, the only way she could hear clearly was to turn on speakerphone and hold it up to her hearing aid. She recently got a ReSound Smart Hearing Aid, which pairs directly to the phone, and now has no problems at all with hearing and understanding phone calls. It's certainly costly, but now she can hear calls more clearly than ever.
From the lockscreen, you can double-click the power button to immediately open the camera.
Just looked and they also work for cell phones actually! CapTel app