Call failed

This seems like a carrier issue either at your end or the other end.

It is my experince that some Carriers apply priority filters to connect to preferred customers and Carriers depending on which Carriers are on good terms with them.

Please Contact your Carrier (Cellular Service Provider). The agency who issued the SIM to you and to your some contacts. I would suggest you to lodge a written complaint.

This seems surely like a carrier issue. You need to contact your Carrier (Cellular Service Provider), the agency who issued the SIM to you.

Call Failed or Dropped Calls / Disconnecting

In telecommunications, a dropped-call / Auto Disconnecting Calls / Call-Failed Muted conversation are mainly due to technical reasons / fault at the carrier's end. At times the calls are cut off before the speaking parties had finished their conversation and before one of them had hung up (dropped calls or Call Failed or Auto Disconnection or muted calls).

Few Reasons Your device Keeps Muting / Dropping / Disconnecting Calls or encounter Call Failed.

• Your Signal Is Weak.
• It's Raining (Cats & Dog)
• The Cell Tower Is Overloaded / Heavy Traffic (Telecommunication)
• You've Misunderstood Your Signal Strength
• You are moving and the signal weakens while changing from one tower to another

Options Are...

• Turn on Airplane mode, wait for 10-15 secs and turn off airplane mode.
• Restart your iPhone (Restart your iPhone, iPad, or iPod touch - Apple Support)
• Force Restart your iPhone )Force restart iPhone - Apple Support)
• Check you Battery health
• Take the SIM out and Put it back again
• Change you SIM, if it is damaged
• Time to contact your Carrier / Change if required