Here in the USA, only T-Mobile and MVNOs which use the T-Mobile network can register with IMS and enable calls using VoLTE. SMS, MMS and data also work on these networks. SMS, MMS and data work with AT&T and any MVNO that uses AT&T. Keep in mind, both T-Mobile and AT&T maintain white lists for phones. MVNOs are permitted to use their own device whitelists for the most part. Puremobile (AT&T GSM-A) works great for data, SMS and MMS. US Mobile (on the white T-Mobile GSM-T SIM) works great with the Xperia 10 ii and 10 iii for data, SMS, MMS and calling. The Xperia 10 iii has very limited service outside of major metropolitan areas due to lacking support for most GSM-T bands used for service coverage in suburban, rural and indoor locations.