will I be able to use my iPhone 5C as a phone if I ignore the IOS 13 upgrade?

An iPhone 5c cannot be updated beyond iOS 10.3.3. So you won’t have to ignore anything as your device will never get an update notification for anything newer than 10.3.3. Just keep on using it as you do now.

Certainly. Your phone will continue to function just as it does now other than for what features you won’t have in iOS 13 which by the way, you can’t download on your phone anyway.

An iPhone 5C is not capable of getting iOS 13. You can continue to use your phone as you are now doing so.