Even though your iphone is Jailbroken, it has to be unlocked as well
It really depends on what iphone you have, for me I have the 2g Iphone and my friend downloaded boot neuter from Cydia, (I think) and i put in a tmobile/ AT&T Sim, run boot neuter, restart it, and it works
Another thing is, make sure you're putting the sim in the right way, try the sim in all of the positions you can, because I put it in a few times and it didnt work cause i had it upside down, on the wrong side, etc
good luck!
