Sometime in Mid-2009 YouMail quietly introduced DropBox. DropBox lets you send email notifications from other voicemail services to YouMail. I use the same service from PhoneTag and it is actually pretty useful. YouMail DropBox converts the voicemail notifications sent via email from another service provider when it is sent to YOURNUMBER@my.youmail.com (where YOURNUMBER is your 10 digit phone number).
The email notification from your service provider must include the audio file attached. Currently YouMail DropBox supports: Vonage, MagicJack, Google Voice, RingCentral, Phone.com, eVoice, OOMA, and Packet8.