Changing the default country of your account
Your default sending country determines how your mobile numbers are validated, as well as which international prefix is automatically added to the front of any contact numbers added to the recipient field or contact profiles (unless a prefix is already present).
To change the default sending country, click on your profile name in the top right, followed by Profile in the drop-down. The default sending country can then be switched by selecting the relevant country from the dropdown list, in the "My Details" section: