One certain person can't hear me when I call. I have an iPhone SE and when I tried calling my
Boyfriend, he couldn't hear me. I called my
Mom to see if it would happen but she COULD hear me. I could hear him clearly. This has never happened before and we've called multiple times. Why can he specifically not hear me? He used his phone earlier and was able to hear other people. What should I do to fix this?
First, just try again, make sure it wasn't just a glitch.

Then, to fully test...  

Reboot all 3 phones, yours, boyfriend's, mom's.  

Ring each person to test - that's 6 phone calls - you to mom, mom to you, you to boyfriend, boyfriend to you, mom to boyfriend, boyfriend to mom.
That last couple might feel odd, depending on how well they know each other, but it's the only solid test to see if it is specifically just an issue between you &amp; your boyfriend.

If only one/two of those tests fails [the expected ones], the next test would involve each person travelling far enough that they are certain to be using a different phone mast &amp; repeat the process.