I am curious why there are no well-known services that replace phone verification despite the fact that so many products spend much money on verification of their users' phones...
I see two possible directions of solutions:
1. Technological: deep understanding of android and ios possibilities to somehow connect to user's sim-card and verify phone. Or in general, doing it with any other protocol or request that does not require a call or SMS.
2. Iterative with existing options: to create a product that tries verifing via sending free imessage (if it is iOS). If it doesn't work, it tries verifing via What's up and so on. If nothings works, it uses the most expensive way - sending SMS.
Can you, please, recommend some technologies or services that are now state-of-the-art solutions of this problem?
Between U2F, bank-issued eIDs, smartcards, SIM applications, blockchain-based solutions (think Metamask), keybase, email links, anything around SAML, NTLM and Kerberos/RADIUS there's quite a lot of options possible.
As for sign-on through Whatsapp, a lot of companies were doing it since 2012 or earlier, first checking if SIM is online (HLR -> IMEI) to confirm it was not replaced since last login, followed by trying to find active online session through 3rdparty gateways and using it.
AFAIK most of these are gone or changed their products lineup by now, engageSPARK and messagebird seem to be alive though.