General Rule:
Admins (and other members) cannot see your phone number if your personal Telegram privacy settings are configured to hide it. This is the default and most common behavior.
Specific Scenarios:
Your "Who can see my phone number?" Setting:
If set to "Nobody": No one, including group or channel admins, will be able to see your phone number on your profile within the group or channel. This is the most private option.
If set to "My Contacts": Admins (or any other member) who already have your phone number saved in their personal phone's address book AND have you as a mutual contact on Telegram might be able to see your number. If an admin does not have your number saved, they won't see it.
If set to "Everybody": Your phone number will be visible on your profile to all members, including admins, of any group or channel you join.
Groups vs. Channels - Member List Visibility:
Groups:
In smaller groups, all members (including telegram number database admins) can typically see the full list of members.
For larger groups (over 100 members), group admins have an option to "Hide Members" (under Group Info > Edit > Members > Hide Members). If this option is enabled, only admins will see the full member list, but their ability to see phone numbers still depends on each individual member's privacy settings. Even if an admin sees a member's name in the list, they won't see their number if the member has set their privacy to "Nobody."
Channels:
In channels, regular subscribers cannot see the list of other subscribers. This is a core privacy feature of channels.
Only the channel owner and administrators can see the list of subscribers. However, similar to groups, their ability to see a subscriber's phone number depends entirely on that subscriber's individual privacy settings. Admins do not gain special privileges to view hidden phone numbers.