Your best plan of action first, would be to go outside where you know your signal is stronger, try rebooting the phone and see if it connects. Also take out the SIM Card and put it back in the phone.
If this does not work, try another cell phone in your house if you have one or enable WIFI calling on the device and see if a call goes through. I know in my apartment my husband and I have the setting enabled as I can get a clearer call than if I were in place without a good signal. Has not failed us yet.
Otherwise something may be faulty in your phone and you'll have to make an appointment with the Genius Bar at the Apple Store for more help.
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