By providing V-mail capabilities to your ATA 186, you are jumping forward into network elements which will deal with both issues:
1. Call forwarding and routing. From, To the V-mail in case of busy not available etc...
2. The V-mail itself, centralized entity that will receive the messages and will play them only to the proper caller
The other question you must answer is which call signaling you are using? I will suggest using SIP since its implementation is much easier then the alternative H.323
Know after I put together few items to think of, the UMS systems that I have experience is from Iperia in conjunction with Cisco SIP Proxy
I hope that help
