Check the exact symptom first: no service or SOS points to the mobile line, while failure with one person points to that number or contact. Preserve the alert and test another call before changing settings.
A failed call is not always a microphone or speaker fault. Read the status bar and the exact alert, then test whether the problem affects outgoing calls, incoming calls, every number, or one contact.
Separate service from a single destination
If the iPhone shows SOS, No Service, or Searching, check the active cellular line in Settings > Cellular and look for a carrier outage or account suspension. If calls fail only for one person, dial the current number manually and review blocked contacts rather than resetting the whole phone.
Turn Airplane Mode on briefly, then off, and restart the iPhone. Check for a carrier-settings update and supported iOS update. Do not remove an eSIM unless the carrier confirms how it will be reactivated.
Review call routing and line settings
Open Settings > Apps > Phone. Check Call Forwarding, Silence Unknown Callers, and blocked contacts. Focus modes can silence notifications but do not normally cancel the carrier call itself; distinguish a missed ring from a call that never reaches the line.
With Dual SIM, verify the intended line is on and selected. Test voicemail or a known local number and note the time and alert. International, premium, and roaming calls can have plan restrictions that ordinary local calls do not.
Escalate with evidence
Success means an outgoing call connects and an incoming test rings on the intended line. If service bars are present but all calls fail, give the carrier the line, location, timestamps, error text, and whether cellular data and SMS work. Contact Apple when the carrier confirms provisioning and the device still fails. Never publish your phone number, eSIM QR code, account PIN, or call records.
Primary sources
Interfaces and policies change. We recheck the steps above against these sources and show the verification date at the top of the guide.