Try a full restore and if that doesn't fix it take it to an Apple store. My wife's iPhone 4 all of a sudden completely stopped working with any mic (Speaker, built in, bluetooth, headphones).
We made an appointment and had a new phone right away.
did you originally get your first iPhone 4 from Apple or one of the carriers? I'm asking because, I'm wondering if you got one thru let's say Rogers, then you take it to Apple to get a new one, if the new one will be unlocked?
I was at work so I couldn't restore, but I took the phone to the local Apple store after work. 30 minutes later I left the store with a new phone.
DA17, the original phone was purchased through Telus. It's my first iPhone so I'm unsure how to tell if it's unlocked. If you let me know what to look for I would be happy to let you know.
If you don't have access right now with iTunes - just pop in another SIM card from another carrier other than yours, reboot the phone and if you have service, then you'll know that your phone is unlocked.