Neither Signal nor Telegram are federated. You can't have an organizational or private Telegram server. Your account is managed by a party you have no control over.
Signal isn't federated because Signal tried to learn from the failure of XMPP to catch on in any meaningful way, in large part due to that federated nature. However, Signal is open source and its devs have said that anyone is perfectly free to fork it and run their own instances, as long as they remove all Signal branding and servers. In this respect, its unwillingness to allow federation does not negate it being free and libre.
