Using the iPhone’s Field Test Mode allows you to easily find out which frequency your cell phone is using to access your cell phone carrier (usually AT&T). The instructions differ depending on whether your iPhone is using the 3G or non-3G network.
If your phone shows a 3G icon in the top left of the screen:

1. Dial *3001#12345#* on the keypad and press call.
2. Select “UMTS Cell Environment”
3. Select “UMTS RR Info
4. If the uplink frequency is a number between 4132 and 4233, your phone is using the 800/850MHz Band. If the uplink frequency is a number between 9262 and 9538, your phone is using the 1900MHz band.