Cellular communication iphone 7.. problem says its ios updated to 15.3. Says update iphone needed. Scam?

What iOS version is already installed on your phone? If it is iOS 15.5, the message is a scam.

If iOS 15.5 is not installed on your iPhone, and you want to update the software, you can do so as follows:

Home Screen > Settings > General > Software Update. Do not use the link in the message or email.

It's not clear where you're seeing this message.

If it's coming from some SMS message, then it's almost certainly a scam.

But if it's coming from the OS, it may be you need a carrier update to work with your cell provider.

Without more detail it's not possible to advise further.

You can't get 15.4. You can get 15.5.