I thought it I could specify any URL - including the twimlets used in conference calls - in the Play verb of a TwiML file. However, the following does not work...
http://twimlets.com/holdmusic?Bucket=com.twilio.music.classical#&Message=please%20wait
... and neither does the twimlet when I strip away the arguments...
http://twimlets.com/holdmusic?Bucket=com.twilio.music.classical
Does the Play noun work differently?
<Response>
<Say>Please wait while I work on your request</Say>
<Play>http://twimlets.com/holdmusic?Bucket=com.twilio.music.classical&amp;Message=please%20wait</Play>
</Response>