What people think is cheating is up to them. Some people find watching porn less bothersome than listening to a voice that seems to be interactive, and others might find the opposite true. Some find it all problematic. There's no right or wrong here.
You do seem to have some insecurity, though - just because your partner likes one voice doesn't mean he doesn't like yours. Just because he finds one woman attractive doesn't mean he doesn't think you aren't the hottest thing to ever land in his bed, or that he doesn't realize how lucky he is that you're there.
Someone who does that kind of voice work is a professional. They obviously have a certain talent for it, one that most of us "regular" people won't have. I certainly don't have a phone or audio sex voice.
Maybe he needs to express his feelings about how he feels about you. Maybe you need to work on self-esteem. Maybe both.
In the meantime, have you communicated your feelings about all this to him? Are you trying to find out if it's cheating to put a stop to it to just spare your feelings, or do you have strong feelings that this is cheating? Again - no right or wrong, just things to think about.
I'm not a big believer in "micro-cheating". I think things are cheating, or they aren't. If my partner wouldn't do it in front of me, or won't share it with me, and other people are involved somehow, there's at least some level of dishonesty there. I'd like to know why - are they just a liar, or am I making it hard to share things, or what.
Also, I notice on your profile that you are 15. Any photos you send to him are illegal, given your age. How old is your partner?