I put in *#21# in my smartphone's dialer, and it says everything is forwarded?! Should I be alarmed?

Short answer: No. 

Long answer: GSM phones let you put in these dialer codes to change some settings. The codes, collectively, are known as USSD (unstructured supplementary service data) codes, sometimes called "quick codes". These settings are NOT in your phone, but on your mobile carrier's computer as a part of your account. 

Generally speaking, USSD codes are used to add or remove service, or change some settings for the service. Some carriers use it for payment notification and payment status. Actual active codes are dependent on the carrier. As this is often used to just show the status of something it's also called interrogation codes, or feature access codes. 

So what does *#21# actually do (note: it's actually asterisk - hash - two - one - hash)? It shows the status of call forwarding set to the five types of calls you get: voice, SMS, fax, data, and system. (NOTE: some phone vendors only show voice, SMS and data, not fax or system)

And in all cases, it *should* say "all forwarded". Because, remember, these are settings for your account on the carrier's computer, NOT YOUR PHONE! Of course you want all those forwarded TO YOUR MOBILE PHONE! 

Some unscrupulous antivirus providers, to drum up sales of their products, have articles on their website with alarming titles such as "What number to dial to see if your phone is being tapped". I am NOT linking to that crap content. 

This misinformation was passed on by alarmist content creators who had absolutely NO technical background to verify what they are passing on. As a result, such misinformation are passed onto people on Tiktok, Instagram, Facebook, and other social media such as Reddit. 

USAToday did a factcheck on this very issue. Their conclusion is clear: false. This code only shows if a phone has call forwarding enabled. It does NOT tell you anything about a security breach. 

If you are scared, you can enter another code to disable call forwarding. You may lose features like voicemail, but whatever lets you sleep at night. 

In fact, Howtogeek has a list of the common short codes (for iPhones), and most carriers will have their own listed at their own websites (albeit, may be listed under feature codes, short codes, or access codes)

Don't believe everything you read, even on an alleged "reputable" website.