My UK HTC Chacha phone running 2.3.5 since the last few months shows a notification since the last couple of days that there is a software update available. But the menu only says that it's a bit over 7MB to downloads, it doesn't specify what software update this is which version -- how to find out the details?
Your carrier is responsible for sending OTA updates. Check their website, call them and ask, or try searching for your carrier's name and "Android update".
The ChaCha does the actual updates check by contacting some webservers (*.htc.com) which are listed in build.prop. All released ROM updates are listed at http://forum.xda-developers.com/showthread.php?t=2094100 but with a 7MB download it must be something else.
The over 7MB to download part could be that the phone is set in a way that downloading files >7Mb would require a Wi-Fi connection.
To avoid data wastage many phones have this feature to enable warning/limit downloads over the mobile data usage. This settings can be managed under the settings options of your Mobile Data option.
Just by un-checking the option you can easily download the update OTA