go into your phone app, type in "*3001#12345#* and tap call
it'll bring you to the field test app
hold the power button and shut down your device
when you turn your device back on, it'll be the signal numbers
if you ever want to return it to normal, just go into the phone app, enter those numbers and tap call again. it'll bring you to the field test app, just press your home button and everything will revert back to normal
note: this can also be performed on a non-jailbroken device!