I am also from Russia and I have the same problem.
When sending OTP to a Megafon mobile phone, an error occurs: "An internal error has occurred. [Error code:39]"
When sending to an MTS phone, Firebase says that the OTP has been sent, but SMS are not delivered to the phone.
When sending to a Beeline phone, everything works fine: OTP has been sent, delivered, Firebase verification is successful.
Have you found a solution to this problem? What alternative to Firebase can be used in Russia?