You can also checkout Message Central for user authentication. Lesser expensive than Firebase and works pretty well.
You can simply use an SMS based verification. Multiple third party API providers for the same like Message Central, Twilio etc.
CPaaS platforms like Message Central also provide some such solutions.
This also in turn affect which channel do business communications use
Had a good experience regarding this with one of the SMS API providers - Message Central.
There has been an increase in SMS phishing attacks too.
You can try Message Central's SMS APIs. They can also give you customized rates. The team is super helpful.