check out the thread I've started
here It seems that PUSH is broken on hacktivated 2G iPhones. Because it's unlocked and jailbroken the iPhone keeps trying to connect to PUSH servers but it can't because there is no plan on the phone ( I suppose, e.g. I have a simple phone plan in Belgium). Therefore this not only wastes battery since it's always trying to reconnect but it also keeps your signal busy resulting in dropped or missed calls and delayed SMS's.
So to solve your problem first install a PUSH app like tap tap for example so that you enable the PUSH setting in your settings. Then turn PUSH off both in the main settings and in the Mail,Contacts,Calendars section, turn the push in "Fetch New Data" off. That solved most problems for me. Hope this helps, and good luck.
Alex