afcovert uses the superior inbuilt AAC converter. FFmpeg can do this as well with the right arguments but you have to dig them up and the quality is capped to a lower value than you can get with afconvert.
You are using Opus on most VoIP/call providers.
Meanwhile, AAC, MP3, OGG... are history today.
No. Opus support in the PSTN is still unfortunately almost non-existent. Adoption is growing but the pace is very slow. Source: me, working in the European telecommunications industry for over a decade, specializing in telephony and SMS.
I like opus on VLC for youtube-dl rips but if I’m buying music I want lossless uncompressed.