The Signal messenger and protocol.

"Third Parties. We work with third parties to provide some of our Services. For example, our Third-Party Providers send a verification code to your phone number when you register for our Services. These providers are bound by their Privacy Policies to safeguard that information. If you use other Third-Party Services like YouTube, Spotify, Giphy, etc. in connection with our Services, their Terms and Privacy Policies govern your use of those services."

We work with third parties to provide some of our Services.

What type of Services? And do Signal use our contacts or something private like that for third-parties?