Hi duffyl123,
It's possible that call forwarding is enabled, as this would cause the behavior you've described. If you have cellular service through a GSM network, this may have been set up on your iPhone. You can check by going to Settings > Phone. You will also see a phone icon with an arrow in the status bar if call forwarding has been enabled from your iPhone. There's more information here: Set up call forwarding and call waiting on iPhone.
If call forwarding hasn't been set up from your iPhone, check with your carrier to see if calls are being forwarded from their end.
If the issue persists, the steps in this resource may be helpful: If you can't make or receive calls on your iPhone.
Contact your carrier
When you contact your carrier, make sure that:
• Your account is set up to use your iPhone.
• There aren't any localized service outages.
• Your account doesn't have a billing-related block.
• Your calls don't have errors on the carrier system.
Update your software
1. Check for a carrier settings update.
2. Check for an iOS software update. Some updates might require a Wi-Fi connection.
Check your iPhone settings
1. Turn Airplane Mode on and off. Go to Settings and turn on Airplane Mode, wait five seconds, then turn it off.
2. Check Do Not Disturb. Go to Settings > Focus > Do Not Disturb and make sure it's off.
3. Check for any blocked phone numbers. Go to Settings > Phone > Blocked Contacts.
4. See if Call Forwarding is turned on. Go to Settings > Phone > Call Forwarding and make sure it's off.
5. Check to see if Silence Unknown Callers is turned on. If the setting is on, a caller must be listed in your Contacts or Recents. Or they must have shared their number in Mail or sent you a message using that number in order for their call to go through. Add their number to Contacts to ensure the phone rings. Calls from some third-party apps might not go through.
Check your network settings
1. Reset your network settings. Go to Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings. This will erase all current saved settings, including Wi-Fi passwords, preferred networks, and VPN settings.
2. Try to make or receive calls in another location.
Best regards.