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Whenever my son attempts to send an SMS he gets the above error, any ideas? He onlys texts full mobile phone numbers, no short numbers
The first thing to check is he has the correct message centre number, this is +447785016005, check he has messages set to text and is not sending as chat here: Messaging Features
Most probably the phone has temporarily lost it's connection, sometimes the SIM can be a little behind and needs to be updated on the network. Ask him to complete a manual network scan: settings > connections > mobile networks > network operators > manual > when the scan completes, select any other network, let it reject the request and reselect Vodafone. Follow this will a reboot.
If the problem persists, he can order a replacement SIM for the number here: SIM swap
Further information for his phone here: Device Support