You can use the silence parameter in conjunction with the machine_detection feature of the Call API to enable your XML to detect whether a call is answered by voice mail. When silence is set to true, as soon as the voice mail finishes speaking, and there is silence for minSilence milliseconds, the next element in the XML is processed, without waiting for the whole period of length seconds to pass. Example Request 
from plivo import plivoxml

response = (plivoxml.ResponseElement().add(
    plivoxml.WaitElement(None).set_length(10).set_min_silence(3000)
    .set_silence(True)).add(plivoxml.SpeakElement('Hello')))
print(response.to_string())
Response 
<Response>
    <Wait length="10" silence="true" minSilence="3000" />
    <Speak>Hello, welcome to the jungle.</Speak>
</Response>