For Sprint they have 3 types of call forwarding, specifically call forwarding unconditional, that can be set by dialing *72 and the 10 digit number forwarded to, but this cost $.20 per minute. (all calls are forwarded as soon as they are recieved, ie they arent initally recieved by your device)
Then there is call forwarding busy and call forwarding no answer, which the last would be the one you would want for a 3rd party voicemaill solution such as youmail. To start these dial *72 + forwarded number, and *74+forwarded number. These may or may not charge the $.20 per minute(sorry unsure), in this topic that would be the time it takes the caller to leave a 3rd party voicemail message. These all can be stopped via star whichever you did(72, 73, 74) and then 0. I had used you mail for a while on verizon and did the call forwarding no answer with them to youmail and got my voicemail as a email via gmail account in form of a waive file, but with verizon it uses anytime minutes if the message is left during peak time, but not $.20 per minute. Hope this helps you somewhat, would recommend youmail, it's free and you can customize each message per caller if you setup your contacts on their site. Freeked people out when it said sorry, contact name, I'm not in right now, leave a message" Gave up on it as verizon finally got Visual Voicemail. Best of luch to you though.
