According to the documentation available at https://firebase.google.com/pricing, it's only the verification that counts (the SMS that it send to the user), this means that you'll only pay for the registration, not the login, if you don't use the SMS to login.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
