Clean project, reinstall pod and run your app, you will see flag icons now.
As i see, this project seems abandoned so you can find a working pod in my fork, alongside with other QoL improvements (ability to tap on country prefix label to open the picker, for ex.)
Podspec is broken resulting in no flag icons at runtime.
In FlagPhoneNumber.podspec,
s.resource_bundles = {'FlagPhoneNumber' => ['Sources/Resources/**/*']}
must be replaced by
s.resource = 'Sources/Resources/**/*.{json,xib,xcassets}'
Clean project, reinstall pod and run your app, you will see flag icons now.
As i see, this project seems abandoned so you can find a working pod in my fork, alongside with other QoL improvements (ability to tap on country prefix label to open the picker, for ex.)