Basically the JB loses the MMS and FT settings, so what you need to do is restore form a backup (a backup that was not JB) and after the restore, the settings for MMS & FT will be recognized again.
It's just important that you don't have your JB iPhone do a backup first, or else when you restore from backup it'll be restoring from that JB backup which already lost the settings. If this does happen, then you'll need to completely restore.
So what you should be able to do is JB your phone, connect to iTunes and cancel the sync and then restore from backup.