Although Signal is free software, it cannot be built without Google Play Services. So you are "free" to modify it, but only if you are willing to use proprietary software to do so. It might still be preferable to Telegram, however.
See: https://forum.f-droid.org/t/signal-discussion-about-google-play-alternative/95/8 https://whispersystems.discoursehosting.net/t/how-to-get-signal-apks-outside-of-the-google-play-store/808
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