Does my UK, US, or EU SIM card provide free roaming in Brunei?

Brunei is unequivocally outside the domestic roaming zones for UK, US, or EU mobile network operators. Consequently, activating international roaming with your home SIM (e.g., from AT&T, Verizon, T-Mobile, or UK's Vodafone/O2) will incur their highest 'Rest of World' international roaming charges, often ranging from $10-$15 USD per day or high per-MB rates. These costs are substantially higher than a dedicated CoreESIM plan, which provides a cost-effective, local data profile.