Adding TOTP would be cheaper in the long run than continuing to pay those SMS rates. I dont think its about any kind of extra hassle they have to deal with. More of terrible NIST standards written by the center for internet security, which is a for profit corporation that apparently nist allows to write all their standards
Financial companies ans banks and stuff have to follow regulations on their MFA method. That why you can't just use any OTP authenticator and are stuck with email/SMS.
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