Your answer was the right one! I have confirmed that by selecting [App Store]-[My Icon]-[My Account]-[Country or Region] on the iPad and specifying the desired country, the country code of the destination (Android) is not automatically changed and the message can be sent correctly. However, when sending a message to an iPhone, the message is usually automatically sent as an iMessage (blue message), but it is sent as an SMS (green message). In addition, the country code of the destination will be automatically changed to [+44] and the message cannot be sent. I have specified [+ country code] for both Android and iPhone. Thank you for your help.

Thanks for your response and awareness. Is the "device" in this case the destination device? Or is it the device that sends out the SMS?