Pick a Username
Claim custom username in Telegram
Turn off message preview
On Android smartphone, there are two ways to disable Telegram notifications. First, using the Telegram app. And second, through the Android settings.
I prefer using the Telegram app as you can disable sound, previews, or private message notifications.
Steps to follow: Open your Telegram app, and click on the top-left (3 lines) to see the menu. Here, click on Settings -> Notifications. The first section is for Private Chats. As you scroll, you will find another section: Groups and Channels.
You can click on Notifications to mute it forever, 1 hour, 8 hours, or 2 days. Or you can selectively configure your notifications. For example, if you turn on Message Preview then you will get notified without the message content.
Turning off notifications for individual contacts
Telegram appears to have recently added some settings to toggle synced contacts as well as delete existing synced contacts. They can be found under Settings > Privacy & Security > Data Settings > Sync Contacts and Delete Synced Contacts respectively.
A fair guess of what Sync Contacts does is it toggles syncing only for the device in question, while Delete Synced Contacts deletes all synced contacts from Telegram servers (Use at your own risk).
Keep in mind that the Sync Contacts toggle only allows controlling syncing of contacts on a device level, which may or may not suffice depending on your use case. The vast majority of users with a primary device would likely have all their contacts synced with Telegram servers, which is largely the cause for the automatic creation of new contact conversations in their chat lists – not that they could help, that’s just how it’s been designed to behave, with no option for disabling.
Should the two new settings still prove inadequate for your use case, or if you’re using older device(s) with a version of Telegram that does not include them, you may then wish to proceed with the following guide
Change your phone number on Telegram
While it doesn't happen too often, you might end up changing your phone number somewhere along the line. Even though you are registered with your old phone number, you can easily swap out your old number with a new one. Just go into Settings and tap on Phone number.