You're thinking of SS7, which is how international voice carriers interconnect (can also do SMS). SS7 was architected around a limited number of trusted participants (national monopoly carriers), but now there are a huge number of participants but there's no structural protection against abuse (although some carriers are able to address some abuses through ad-hoc means)

4G (LTE) at least has mutual authentication between the carrier and the user, so that's a meaningful security improvement.