According to a regulatory modernization initiative by the Nigerian Communications Commission (NCC),  the Association of Licensed Telecoms Operators of Nigeria (ALTON) has announced the use of harmonised short codes (New USSD code) across all networks.
The Association stated in a press release that was made available to Nigerians that the harmonisation of shortcodes aims to establish a simplified process for common shortcodes across all networks providers, making it easier for Nigerians to be able to use just one code across all networks. 
This uniform code works on all network provide like MTN, Airtel, 9mobile and Glo.
Though the old ussd codes are still working. We all grew up knowing these codes and now we have to learn a new set of ussd codes. I really have no clue why they are constantly changing. I know how difficult this will be.
That’s doesn’t change the fact that you should not be familiar with the new ussd code as old codes will seize to function as at May 17

New NCC Harmonized USSD Codes 

The following are the latest Harmonized codes that has been approved by NCC.

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Services NCC Harmonized Approved Codes
Call Center/Help Desk 300
Voice Mail Deposit 301
Voice Mail Retrieval 302
Borrow Services 303
STOP Services 304
Check Balance 310
Credit Recharge 311
Data Plan 312
Share Services 321
Data Plan Balance 323
Verification of SIM Registration/ NIN – SIM 996
Porting Services (MNP) 2432
These are the NCC approved USSD codes. The above new ussd code works on MTN, Airtel, 9mobile, Glo, etc
The old codes, which the telcos were still using would however, coexist with the new codes for a period of time before being finally phased out as of May 17, 2023.
You can avoid stress of having to memorize all telco service codes with the harmonized shortcodes.