Android, by default, logs your last 500 calls. There is an app in the Android Market to help backup and restore your call logs on another phone or after a full reset. A very small app, but very useful.
Call Logs Backup & Restore is a free app that lets you do exactly what it says to. It creates a backup of your call logs and you can easily restore them back later on the same phone or a new phone.
Download Call Logs Backup & Restore from Android Market