You can use the Wait element to aid leaving voice mails on answering machines by adding an extra parameter called beep and setting it to true. Example Request 
from plivo import plivoxml

response = (plivoxml.ResponseElement()
            .add(plivoxml.WaitElement(None)
                 .set_length(10)).add(plivoxml.SpeakElement('Hello')))
print(response.to_string())
Response 
<Response>
    <Wait length="10" />
    <Speak>Hello</Speak>
</Response>