Re: Can you change nokia windows phone to straight talk by buying a sim card
Yep... Never put your IMEI or Serial # on the web...
If the phone is branded (has a carrier name on it like AT&T or T-Mobile) it has to be carrier unlocked for you to switch it anywhere. Carrier Unlocked or Factory Unlocked (manufactured as unlocked and with no branding) phones can run on Straight Talk with the correct SIM style. Check here for more details: SIM Cards, Bring Your Own Phone: StraightTalk Wireless.
Re: Can you change nokia windows phone to straight talk by buying a sim card
SWAPPA tells me your phone is an AT&T branded Lumia 520 and your phone is not blacklisted...
It needs to have an AT&T style Straight Talk SIM inserted. It may not need to be unlocked to work. YMMV. You could test it if you have a friend on ST with an AT&T compatible SIM they will let you borrow for a moment...