I suspect that relying on IDs owned by another system can have its drawbacks. Maybe it's not likely that an entire country is forced to change phone numbers. It's still possible to lose a number for various reasons (most entirely benign). An oppressive government could definitely disrupt communication of suspects by forcing the phone company(-ies) to terminate their contracts and unlist their numbers.

(But IDs being piggy-backed to a different system which is not easily disrupted on a mass scale does have some advantages, too.)