my phone says "call failed" when I try to make a call

(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2)  If the above does not work then - Contact your carrier to ensure that you have active service provided to your iPhone.

Axel F.

my phone says "call failed" when I try to make a call

unconditionallove Said:

"my phone says "call failed" when I try to make a call"

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Troubleshooting Call Failed:

A. Installing a Cellular Update:

It may be prompting you for a cellular update to install. So, go here for that. Update your Carrier Settings Manually on your iPhone or iPad - Apple Support

also...

B. Troubleshooting "No Service":

"No Service" is shown on the top-left corner of your screen? So, go here to fix that: If you See No Service or Searching on your iPhone or iPad - Apple Support