Go into the main menu, then Settings, then Security.
It will probably ask you for a security code, the default is the last 4 digits of your phone number.
If your phone number is 555-555-1212 then the security code should be 1212.
In the Security menu, you have an Erase Contacts option.
Also, there is a Reset Default. After running the Erase Contacts, you can run the Reset Default and that will erase everything from the phone, and make it factory default.

I am giving away this older phone {LG lx 5550} and I want to do a mass delete of my contact list before I do.

derkster