Interestingly they can also effectively communicate well before they can vocalise words. A baby can sign reliably at 7-9 months but typically won't be able to speak until 12 months.
Yes! Signing (albeit with a small vocabulary) was surprisingly successful with my infant daughter, over 20 years ago. I was initially skeptical, but it worked really well. Not sure if it reduced my infant daughter's communication frustrations, but it certainly helped her parents feel less frustrated.