Use IAX if you want, it’s probably gonna work just fine for you, it didn’t for me though.
I was using it initially to connect Asterisk over the internet but suddenly started to run into show stopping problems that I couldn’t fix. That’s where I found that iax has poor debugging facilities with logs that are hard to read and few people understand them.
Then I thought why bother with long end of life software anyway and went to sip and won’t be going back.
The only real advantage of IAX is that you have signaling and media over the same port which makes NAT traversal easier.