Why do sender IDs change in international SMS messages?

When crossing an international border, the SMS message’s sender ID may change. 

The Sender ID is controlled by the carrier, who’s priority is to ensure messages are delivered to their intended destination. Carriers may change the sender ID for outgoing SMS messages to improve deliverability. If the carrier changes the sender ID, any responses from the customer to the altered sender ID will not be received.

The best way to preserve the original sender ID is to provision numbers from the same country of the recipient for which you intend to send messages.