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Help ZTE Merit 990g Straight Talk - SIM network unlock PIN

Replacing the SIM card (Straight Talk -> T-Mobile).

Initial Message: SIM card invalid

Many thanks to all contributors in several forums for all of the education and contributions of time/talents/work of which I have gleaned measurable experience.

1) Got the NCK/NP Code but could not get a menu option to enter it.
2) Custom ROM (stock minus bloat).

Current Message: SIM network unlock PIN menu
1) Tried NCK/NP Code -- Not accepted and "attempt counter(10)" decremented.

Lock Screen Message: Phone is SIM service provider locked

Config Changes:
Rooted, sd-ext4, CWM 5.x, Custom ROM

Current Config:
Android Version: 2.3.4 (after ROM -- was 2.3.5)
Kernel Version: (zte stock)
Build Number: "Custom ROM"

1) Config changes produced a "Unlocking Menu" that requires a PIN which is apparently not the same as a NCK/NP code.

1) Where is this offending App (ROM/Kernel)?
2) Where is this "unlock counter attempt" file?
3) Is there a maint code to bypass this menu?
Which SIM did you use when you first got your Merit? If you started with the one that came with your phone, odds are it's permanently locked to ST; but if you initially used a 100% AT&T-compatible SIM (including ST/Net10 AT&T BYOP) it should be unlockable. (Same goes for Net10 Merits; that's why I used my existing Net10 AT&T BYOP SIM with my Net10 Merit.)

Also, some online unlocking resources suggest using an 8-digit NCK/NP code from their ZTE Blade calculator on Merits that won't unlock with the usual ZTE NCK/NP code.

Finally, even if it's unlocked you may not get more than 2G data unless T-Mobile is now using the 1900 band for 3G in your area. (IIRC that was supposed to have been done nationwide by now, but I'm not 100% sure if it's happened everywhere.)
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Thanks for the reply. This phone was a throw away from one of my daughter's friend as a replacement for my lost Nokia ($19.95). It had a ST SIM inside. I just tried a AT&T SIM and got the same message as the T-Mobile .. "Phone is SIM service provider locked" as opposed to "SIM Card not provisioned" when inserting ST.

Tried all of the researched unlock codes/generators I could find, to no AVAIL (pun intended).

As you probably can tell, 2G is a grand leap for me! I refuse to pay $25 to unlock this phone via the so called "Unlock Client" and since this has been touted to work for some people, that means that it is possible. I don't mind hacking this phone as it provides me with a challenge. I am seeking some pointers to continue my journey. I guess the firmware is next.
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