SIM

I have a pretty new (to me) iPhone 6s

First, Restart your iPhone, iPad, or iPod touch - Apple Support

If that fails, I have needed to OPEN - then close - the SIM tray TWICE already > Remove or switch the SIM card in your iPhone or iPad - Apple Support

Just Open then Close the tray

Tell me

hazar ~ What device is this occurring on?

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Writing an effective Apple Support Commun… - Apple Community