Before travelling to the US, ensure that you have activated roaming services by dialing 200 for PostPay. If you had activated, on arrival you will be automatically switched to the available networks. If it does not automatically switch, manually select the available network on your device under network settings. AT and T and T-mobile are the official Safaricom roaming partners in the US.
For roaming inquiries, can contact Safaricom via email to firstname.lastname@example.org using the email you registered your post pay account with to have your roaming line activated.
You can also call Safaricom via +254204272200 or +254722002200 (calls are chargeable)