“SIP was initially released in 1999, and was designed with the assumption that each device has its own globally routable public IP address.”
A home phone network now costs a few IP phones, an Asterisk box and about $5 a month to VOIP.ms for a real number. The hard part is NAT, because SIP bet on IPv6 arriving and lost. SIP only negotiates where the audio goes, and behind a home router it negotiates the wrong address. A protocol from 1999 still breaks on the network setup every house has.