There was a multiple years of gap between rollout of voice call specification on LTE(VoLTE) and launches of first featurephone operating system supporting it. Android and iOS were only implementation available for a while. For this reason, practically all "featurephone" style phones that supports voice call, except very few, runs AOSP. At the point where your product runs AOSP, you might as well launch it as a low end smartphone, which is what a lot of vendor do.