iPhone loses SMSs after software update
Open up "Settings" then tap on "General" and scroll down to the last option "Reset". Tap the third option "Reset Network Settings". This will take a few seconds and then your phone will restart. 

BE ADVISED Doing this will deleted your saved wireless network passwords and wireless network history so you will have to reenter these passwords to reconnect to wireless networks. This option does not delete any of your personal data off your phone.