From what I gather, the phone number is only used for identification of a user account. Can't exactly remember where I came across this information (probably an ama from their team on reddit) but Signal have recently said they are looking for ways to make the above not a requirement for its users.

I don't see this sim lockdown being a problem unless you want to create a new account or for some reason your account has been logged out requiring 2FA. In my understanding, telegram also asks for SMS for 2FA on re-login. Though the 2FA code will be simultaneously sent to any other device you've logged the account in.