There is no government-mandated free roaming in North America. However, major US carriers (T-Mobile, AT&T) voluntarily include Canada and Mexico roaming in many plans. This is carrier-driven, not regulatory — plan terms vary widely and can change at any time.
Inter-Zone Agreements
No government mandate exists. T-Mobile and AT&T include Canada and Mexico in most postpaid plans. Verizon charges for international roaming or offers day-passes. Canadian carriers often include US roaming in premium plans. Mexico's carriers vary. This is entirely commercial — no regulatory protection.