App consuming lot of data due to Get Tag list block

I have an app which select a random user from the database. The problem is that the tag list is much larger because it contains mobile numbers of all users that are registered and will gradually increase as new users register on the App. So, due to this get tag list shows delayed response and consumes approximately 1 gb of data in a few minutes(5 or 6) and this is bad for the users and the developers. I just want to know a different method for doing the task of selecting a random mobile number.

Which db you using ?

@Adhil_Salim once discussed same with me.
Firebase is a realtime database so it is obvious that it will consume more data.
You should try MySql or Airtable.

If using firebase, get Firebase to do all the heavy lifting by using a REST query (web component)