Touching and sliding the lock indicator on your screen will allow you answer incoming call when your screen is locked. If this feature is not working properly please do a soft reset to resolve this issue.
Soft reset:
Remove battery with phone ON.
Replace battery after 10 seconds.
Power phone ON and allow start up cycle to complete.
If issue continues, please send me a private message including your wireless number and the best number to call you back and I will be happy to follow up with you to further assist you. If you prefer, you can contact our Technical Support department at 1-800-922-0204 for immediate assistance.