> No signature on paper can force you to keep talking in police custody. You can stop talking at any time, regardless of what you signed.
Cops are very persuasive when they want to get an answer out of you. Say, they arrest you for whatever, and they can just threaten you to remain in custody for a day... no matter if your pet is alone at home and needs to be fed, or your children need to be picked up from school. Sure, you may call a lawyer to arrange for stuff, but we'll keep your house's keys as evidence...
The freedom to not say anything is utterly depending on circumstances allowing you to do so.