I remember back when I was a kid, before unlimited voice and data plans became the norm, recharging a SIM wasn’t something you could do in a few taps. You’d walk into a small grocery or telecom shop, get your number recharged, and then dial something like *123# to check if it went through. It would show your balance, validity, everything you needed to know. Simple, straightforward.

But here’s the thing, those weren’t just “balance check” codes. That same system includes other codes, and some of them, if used without understanding, can quietly reroute your calls and OTPs without you even noticing.

The functionality behind this is called call forwarding, and it’s controlled using what are known as USSD or MMI codes. The *123# example we talked about earlier falls under informational codes used to fetch things like balance, IMEI, or service details.

The ones you actually need to be careful about fall under call control codes. These can enable features like call forwarding, call barring, or call waiting. All legitimate features, but in the wrong context, they can be misused.

Let me walk you through a scenario I came across recently, it explains this much better than definitions ever could, and we’ll follow it up with how to detect and stop this if it ever happens to you.

There’s a guy named John.

One afternoon, someone shows up at his door claiming to be from a telecom or internet provider, asking if he’s been facing Wi-Fi issues. John says no, and also mentions that he never raised any request for a technician.

The man quickly backtracks, apologizes, and says there must have been a mix-up. He then suggests that John should inform their head office that no issue exists, just to close the request on their end.

To “report” this, he asks John to dial a code followed by a phone number.

John, not thinking much of it, does exactly that.

The code? *21*1234567890#

What John doesn’t realize is that he hasn’t reported anything, he’s just enabled call forwarding on his number, redirecting all incoming calls to 1234567890.

Obviously, that’s just a placeholder number here but let’s break down what’s actually happening behind the scenes.

While it may seem harmless at the start, this code enables what we call unconditional call forwarding. For example, if the code is *21*<phone_number>#, *21* is the command that forwards all the calls, <phone_number> is where these calls should be forwarded, and # executes the command.

This comes from GSM supplementary service syntax, where in *SC*SI#, SC is the Service Code, and SI is the Supplementary Info.

Now that we’re aware of this, how do you actually check and stop this?

Firstly, always beware of inputting any such code without any prior information about it into your Dialpad.

Second, if you want to check if call forwarding is enabled on your phone number, simply dial, *#21#
This MMI code will return the info about call forwarding, both on Voice or Video calls such as in the image below.

Now, suppose there is a number to which your calls are being forwarded, you can stop forwarding using the code, ##21# and this should give you a message informing you that Call forwarding was disabled as in the image below:

I hope this helps anyone out there, and if you’ve reached this point, I would like to thank you for giving this a read. Do share it with your peers and family to make sure they are protected from such scams.

I am building this blog in the hopes that someday we can educate enough people to avoid them from falling victim to such scams, your support can be a game changer in our cause.

Any feedback in the comments is highly appreciated.