I have the same problem on 4.3.2 jailbroken with no bars showing.
I am officially on AT&T, and preserved my baseband just in case I need it unlocked for any reason if I travel overseas.
Would upgrading my baseband solve the problem?
Also, how can I upgrade my baseband? When I update through iTunes, it still gives me 1.59.00
If you do a clean upgrade to 4.3.2 via iTunes, you should automatically be upgraded to the latest baseband that comes with 4.3.2, and not experience any issues like signal bar /MMS / VM.
Usually, the only way to preserve your existing baseband (eg. 1.59.0), is via Pwnage tool (Mac) or Sn0wbreeze tool (Win). If you don't want the hassles of signal bar / VM / MMS not working consistently with running an older baseband on 4.3.2, your best bet is either running 4.2.1 + 1.59.0 bb (if you have 4.2.1 blobs saved via Cydia or TinyUmbrella), or I would recommend doing a clean upgrade to 4.3.2 with the latest bb, and then when it comes time to travel internationally, hope that either the NCK brute force unlock is accomplished by then, or a 3rd party solution like Rebel SIM is still around (given it's legal implications).