Unable to adjust iPhone's volume on incoming calls

For the past several weeks, I have not been able to adjust the volume on incoming calls. The volume is so low that I have to use the speaker, which is not always an ideal choice. I have tried every fix I could find online (and I'm obviously not the only person having this problem), but I still can't change the volume during a call. Even the ring sound I'm hearing while waiting for the other person to answer is very distant.

Any suggestions?

[Re-Titled by Moderator]