Correct, any calls that actually ring on your phone will charge to the phone's service plan, unless of course it is either during free hours (evenings & weekends), or if those calls are coming from either numbers specified on your "Friends & Family" plan or if they're coming from another customer on the same mobile carrier and they have "free mobile to mobile".
Also, a lesser-known cost...if you call forward calls which originally dialed your cell phone number, to let's say your home phone for instance, then those calls STILL get charged to your mobile plan even though the call will ring at your house phone.