Answer:
These types of phrases are referred to as wake-up messages.
Step-by-step explanation:
Wake-up messages are usually used during a speech or in public speaking in order to keep the attention of the audience on what the speaker is saying.
These phrases can be "now listen carefully" and "I'm only going to say this once" and they incite one's attention. They are also an incentive for the audience to concentrate on the speaker's words.