I'm not an expert on the business side of things. If VoLTE can be, and is, used to establish and terminate calls in the PSTN domestically it means it continues

I'm not an expert on the business side of things. If VoLTE can be, and is, used to establish and terminate calls in the PSTN domestically it means it continues the call termination business model. So there's no technical reason that it can't also be the case for international calls, or calls while roaming. There may be other economic reasons, maybe the international market changed and carriers abroad who implemented VoLTE agreed not to charge each other for this, and AT&T didn't want to be part of that. I'm not sure. But just as Skypeout uses VoIP but still bills you for a call and settles after with the terminating network, VoLTE itself presents no technical limitation. I hope that helps.