Third-party service: CIA
The Signal messenger and protocol.
The SMS based pone number verification is one. Another one is google for reCaptcha used to create a signal account. There is also some google dependency for the face blurring. There may be more.
Thanks for reply! I have a Samsung phone that De-Googled %100~ (exclude some important Android depencies based on Google). I restored GMS for auth in Signal. After the auth, I uninstalled it back. There is no Youtube, no Google - Chrome, no Spotify etc. Just FOSSed and de-Googled phone. Do I still using Google Services indirect ways because of Signal? Is it possible to be another third-party sharing except phone verification?
No. Signal doesn't have Google service dependencies. Although, you have other problem here that you probably heard before, using a Samsung phone "de-Googled" won't make you free of Google. You should check if possible to get an Pixel with GrapheneOS.
Signal does have dependencies on Google as they are required to handle push notifications on Android (without constant background usage)
To be honest, I'm not sure. Signal does not have an official f-droid build. They distribute an APK on their website for degoogled devices which allows for websockets to be used as the google notifications systems is not used on degoogled devices, but I'm unsure what it means for the rest of the dependency. Some dependencies can be shipped with the apk so do not require google services to be installed, but an expert should probably shime in to shed some light in this situation specifically.
I believe Signal uses Google Services Framework to send notifications. If you want am even more degoogled Signal, Molly is an amazing project. It's available on Fdroid, possibly using the izzy repository.
It does offer an example of the services they use third parties to provide in the next sentence. So they're obviously giving out the phone number you provided at sign-up to the verification code people. I don't think there's any reason to believe they're giving away your contact list from this paragraph anyway.