How can EE and BT be the only ones to support, surely it’s dependent on the underlying network, not the MVNO? If it’s a feature controlled network side, presumably it could be turned off at whim network side.

TL;DR: not many carriers; not many devices.

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Supported networks by country:

   Germany: Telekom
   United Kingdom: EE, BT
   United States: Boost Mobile
   Thailand: AIS, True