This code dials the extension 2410 at the number 15671234567. The Number element describes the phone number and gives it the sendDigits attribute. To dial the extension a couple of seconds after the number is dialed, we add a few leading w characters, telling Plivo to wait 0.5 seconds for each w. 
from plivo import plivoxml

response = plivoxml.ResponseElement()
response.add(plivoxml.DialElement().add(
    plivoxml.NumberElement('12025551111', send_digits='wwww2410')))
print(response.to_string())
Response
<Response>
    <Dial>
      <User>sip:alice1234@phone.plivo.com</User>
      <Number>15671234567</Number>
      <User>sip:john1234@phone.plivo.com</User>
    </Dial>
</Response>