Try a factory reset. Settings > User & Backup > Backup and Reset > Factory Data Reset. Select Reset Device and then Delete All. This seems to be common practice with a number of other Android devices and should resolve software glitches when upgrading to a new operating system.
As for the 'no hearing aid mode', the instructions I'm reading point to: Settings > My device > Call > TTY mode. Tap TTY Full, TTY HCO, or TTY VCO. This information was taken from an older publication dated July 2014, so it's likely referencing KitKat.
Let us know if these problems persist or if you see an improvement in performance. Feedback is always welcomed.