SS7 was not really designed with any security. It assumed only telcos would be using it and that stopped being true in the 1980's/90's as the bar to entry for

SS7 was not really designed with any security. It assumed only telcos would be using it and that stopped being true in the 1980's/90's as the bar to entry for getting your own SS7 link was lowered. Even if SS7 were retrofitted to support this type of validation it would be negated by the fact that numbers are portable. A number can legally originate from anywhere. Validation will have to occur out of band by some other means or by replacing or deprecating the telco network entirely.