Yeahbut who uses SMS for 2FA on a critical account anyway?
PSNs (public switched networks) use the SS7 signalling protocol which was standardized in 1976 (or thereabouts). In 1976, a telephone exchange authenticated another exchange because they were connected by a length of copper.
That hasn't been the case since VoIP services connected to the PSN.
See this El Reg article https://www.theregister.co.uk/2017/05/03/hackers_fire_up_ss7_flaw/