You can add a key to store last OTP in an in-memory storage like Redis or closured Dictionary in your backend language, you will then update this key with the value of the OTP you want to send out via an SMS service provider. The last sent OTP stored in this key should then be exposed via an additional api endpoint in your service, which you would then call as part of your automated script after doing an action that will make the OTP to be sent.
A more reliable and robust modification of this will be to expose an API that retrieves the OTP stored against each phone number, you must have this stored already of course before you could do the verification in-app. This endpoint must only be exposed within a test environment, that's the precaution to pay attention to, or you make the endpoint available only to authorized user roles.