What to do when a USSD code returns nothing?

It is usually not the phone. Three causes account for most cases, and each has a different fix.

1. The wrong SIM is selected — on a dual-SIM phone you must choose the line when dialling. Pick the right one and try again.
2. Temporary network load — try again shortly, ideally outside peak hours.
3. The code is no longer live — the operator changed it. Get the current one from the official app or the helpline.

The “invalid MMI code” message usually points to the third cause — the network does not recognise the code. Restarting the phone or reseating the SIM will not help, because the problem is not in the handset.

A code will also fail in flight mode or with no signal, because USSD talks to the network. It needs no internet, true — but it does need mobile network, and those are not the same thing. If nothing works on any code, check whether ordinary calls go through: if they do not either, the issue is the connection rather than the code, and the fix is a network problem rather than a wrong number to dial.