Does my existing US (e.g., AT&T, T-Mobile) or UK (e.g., Vodafone, O2) SIM card offer complimentary international roaming in Australia?

No, Australia is not included in typical domestic roaming allowances for US or UK mobile plans. It is not part of any regional free-roaming zones like the EU. Most North American and European carriers, such as Verizon, AT&T, T-Mobile, or the UK's EE and Three, will impose significant daily roaming charges (often $10-20 USD or £6-10 GBP per day) for data, calls, and SMS. Utilizing a CoreESIM offers a predictable, cost-effective alternative to avoid these exorbitant roaming fees.