If you're planning an IP telephony system, voicemail is probably a fairly important component that you should consider...
in detail before you implement. Due to the nature of many common voicemail systems, there are a lot of vendor-specific quirks you'll probably come to know and love. Fortunately, most systems are now capable of communicating with your softswitch with SMDI, which is the Simplified Message Desk Interface.
These quirks become a lot more involved when redundancy is part of your design. The primary reason for this is that SMDI systems connect the voicemail system to the softswitch via a standard EIA/TIA-232 serial cable, which most people still refer to as an RS-232 cable. These serial cables, as you probably know, have only two ends on them. This doesn't make it easy to hook backup systems in place. To make things more complicated, voicemail systems rarely have multiple RS-232 ports.
So... because of these constraints, you want to pay attention to a few details. First, know what method of redundancy your vendor's systems support. It may be the case that they don't support RS-232 "Y" cables, and require you to buy a "data splitter" box instead. Also, if you were planning to put your softswitch in one room and your voicemail system in a different room, you may need to rethink this, because serial cables have some distance limitations. If distance isn't a problem, don't forget to include running the serial cable in your project plan.
Tom Lancaster is a consultant with over tens years experience in the networking industry, focused on Internet infrastructure. He is co-author of several books on networking, including CSSP: Secure PIX and Secure VPN Study Guide, published by Sybex.