well, that's asking quite a lot from ANY system.
1:The biggest downside I see from setting this up through a proprietary system is if the system is forwarding to your cellphone it's eating up 2 lines on your system and the voice or volume quility tends to suffer some.
2: It's not going to pass the CID from the caller to your cell, it's just going to show your office calling.
3: Almost any voicemail can give you external message notification. Most can also play the CID #. Even the ones that have an option to call back, (with one touch) the voicemail is assuming that your on a "system phone"
4: To forward to your cellphone and have the system pull the call back to go into the proprietary voicemail before your cell phone voice mail grabs it could be very tricky and with all the timers set right it usually isn't very dependable.
5: How large of a system are we talking about?
What kind of CO lines,5 or 6 pots, T-1, primes, DIDs, Etc.?
Most of the professional offices I have worked with use an external sorce, I'll e mail you a link, they issue you the numbers, when you call that number it answers with your custom message and provides a menu; IE; what would you like me to do? take a message, find him etc.
If you answer "find him" it trys 2 or 3 numbers, that you provide, at the same time, if no answer then it takes the message and attempts to notify you by your instructions.
This in the long run could save you money by allowing you to simply have a small system in the office without a proprietary voicemail system.
I'm sure we'll get many other thoughts on this.