The recognised training centres are the following:
Does your signal fall away during mobile calls, but your Wi-Fi connection is good?
Consult your operator to know whether “Voice over Wi-Fi” (VoWiFi) is available. This allows you to make calls and send text messages without a mobile signal over the fixed Internet line.
When activated (in your device’s settings), your smartphone will be connected to the Wi-Fi network to make calls over Wi-Fi.
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Telenet/BASE