There are many ways to get your number. Many apps have access to phone numbers, contact info, and more without you ever knowing it has been accessed. But robocalls can just 'say and spray' a swing and a miss is nothing to a robocall. Many use IP phone numbers to call or text.

Google and Chrome have access by default on an android phone. (and probably the biggest culprits of sharing)

Androids tend be more permissive by default.

iOS tends to be less permissive by default but not foolproof.

Check through your permissions on every app.
Usually found through the main settings on the phone.

Some features per app may need to enable them for certain things but they can be toggled on/off per use.

Report calls and texts as junk or spam to your carrier before blocking. Carriers can analyze patterns and warn or block automatically.