It's a regulation problem, not a technical one. In Germany --and probably in other EU countries--, the telco must make sure that a caller is authorized to use a certain number as origin. STIR just shifts that responsibility to some other entity.
If a call originates from an untrusted network, you remove the number or indicate that it is user-provided and probably fake. It would help if phones would properly display the difference (P-Asserted-Id present or not).
If a call originates from an untrusted network, you remove the number or indicate that it is user-provided and probably fake. It would help if phones would properly display the difference (P-Asserted-Id present or not).