Yes, it is possible to customize both the Email and SMS OTP Message that goes out to your users. The steps vary depending on the plan you are on.
For miniOrange Gateway Plan:
If you are on the miniOrange Gateway Plan then click here and follow the steps given.
For Your Gateway Plan:
If you are on the Your Gateway Plan then just follow the steps go to the Settings > OTP Settings Tab of the plugin to customize the message going out in the Email or SMS.