You've already taken significant troubleshooting steps:
1. Resetting your iPhone
2. Changing your SIM card
3. Updating to iOS 15.8.4
Given that:
1. Data (WhatsApp, messages) works fine
2. Signal strength is perfect
3. GSM calls aren't working (caller hears "number not reachable")
4. You receive missed call alerts via SMS
5. SMS sending is delayed (requires 3-4 attempts)
Potential causes:
1. Carrier settings issue: Contact your carrier to verify your account and SIM card settings.
2. Network configuration: Ensure your iPhone's network settings are correct:
- Go to Settings > Cellular/Mobile Data > Network
- Check "Roaming" is enabled (if applicable)
- Reset network settings (Settings > General > Transfer or Reset iPhone > Reset Network Settings)
3. SIM card compatibility: Although you've changed the SIM, ensure it's compatible with your iPhone 7 and carrier.
4. iPhone's cellular modem: Hardware issues might be causing the problem.
5. Carrier-specific issue: Outages or technical issues with your carrier's network.
Additional steps:
1. Check for carrier updates: Ensure your carrier settings are up-to-date (Settings > General > About)
2. Disable/Re-enable Cellular Data: Toggle off and on (Settings > Cellular/Mobile Data)
3. Reset iPhone's cellular settings: Settings > General > Transfer or Reset iPhone > Reset Network Settings
4. Visit an Apple Store or authorized service provider: For further hardware diagnosis
5. Contact your carrier's technical support: To investigate potential network or account issues
Provide your carrier's response and any additional details for further assistance.
Also, consider asking your carrier:
1. Is my account active and enabled for voice services?
2. Are there any outages or maintenance in my area?
3. Can you reset my SIM card or account settings?
4. Are there any specific settings or configurations required for my iPhone 7?
