I am not able to update iOS 15.6 and this issue has caused my iPhone 7 no access to receive any phone calls.

Why are you unable to update the iPhone 7? What message(s) do you see explaining the issue?

Please work your way though these lists of troubleshooting steps:

If your iPhone, iPad, or iPod touch won't update - Apple Support

Updating iOS: common issues and solutions - Apple Community

See if these links help...

If your iPhone, iPad, or iPod touch won't update - Apple Support

Updating iOS: common issues and solutions - Apple Community

I understand how important it is to be able to receive phone calls. We wouldn’t want you to miss anything important. We’ll be happy to help!

It would be great if you can include the error messages you’ve got to this issue (some good tips has been mentioned here). This helps us narrow our focus in finding the root of your iPhone won't update issue. My personal recommendation would be to have you reset your device and start to update your iPhone 7 again. This can be done by going to Settings > General > Reset. Doing so will allow for a fresh restart to your device.

Let me know if you continue having issues. Hope you have a lovely day!

T-Deinem, Newbie at Apple Community

See if this applies -

iPhone 7 Repair Program for "No Service" Issues - Apple Support