I want to put more text in OTP Code message comes flying into my phone.

How are you sending the OTP to Clickatell? Are you using the HTTP Auth method or are you using something else? For the HTTP Auth method you specify yourself the parameter that Clickatell will use to form the sms, so in the Hidden Form Parameters you might have something that looks like this (assuming clickatell are still using the parameter "text" for the message:

text %{session.otp.assigned.val}

The session variable session.otp.assigned.val is the variable that will contain the number generated by the OTP Generate Agent in the Visual Policy Editor, so assigning this value to the parameter "text" is what makes Clickatell send an sms with that value. But you can assign any string to this parameter, for instance this:

text "F5 OTP Code Number: %{session.otp.assigned.val}"