it is a voicemail so no need to be nervous
you can say:
hi my name is [name] calling from [your state] and I am calling on behalf of Brandon Bernard who is an inmate on Federal Death Row. I am asking that you commute Brandon’s unjust death sentence to a sentence of life without parole. Brandon does not deserve to be executed and can still continue to be punished for his crimes, while living out the rest of this life in prison. Thank you