Depending on your current firmware, updates also update the radio's baseband. I was having the same problem for a while until i did a full update. Carriers will always change their network configurations to increase their data speed. Also before you do so, back up your files and do a complete reset all settings from settings/general/reset all settings. It will take about an hour. This will erase any registry file value that could be causing the issue. Hope this helps.
Answer to: iPhone keeps losing service after I dropped it