Log into your Dashboard, expand your account name in the left panel, and open the "Settings" tab.
In the "API keys" setting page, you will see the "SMPP Settings" section, from where you can find an Encoding drop-down menu box for you to select the default character-set encoding to be used:
Do not forget to 'save changes' once you have selected the character-set to be used as the default for your account.
1. This is only applicable for SMPP. Nexmo REST API uses WEB standard UTF-8.
2. This setting only takes effect for SMSC default alphabet (DCS=0). Whenever the data_coding is not 0, it will be replaced with the coding schema specified in data_coding.