Too many hide their documentation. Even if you register on their site as a guest, you can't get it. You usually have to be a customer with a paid support agreement. This includes Sylantro, BroadSoft, MetaSwitch, Acme Packet, General Bandwidth, PolyCom, and Alcatel-Lucent. Your equipment may ship with a CD-ROM of old documentation, if you're lucky.
Others open up their documentation. Cisco and Juniper are in this group. Cisco is notable, because they have documentation on all their VoIP products available online for free. Aastra, Polycom, Snom and Adtran have their documentation available, as well. Of course, Asterisk's documentation -- and source code! -- is available. From an engineer's perspective, this gives them a definite competitive advantage.
I care about this because I spend time configuring and troubleshooting equipment that belongs to other companies. For example, when fictional company name Stratum(3) had problems with their Acme Packet SD, I didn't have access to their support agreement. Because of that, I didn't have access to the documentation on the SD. Stratum(3) is willing to hire me for my expertise, but not willing to give me access to their support agreements.
Closed documentation just makes it hard to get support; it makes products hard to use. The documentation is not google-able. If you hire a new staff member, it's harder for them to get docs on the equipment you have. And if you bought the product in the past, but you're not maintaining active support, you don't have access to documentation on software that you did buy.
Recently, I noticed that BroadSoft Inc has made a bold step toward publishing their documentation: BroadSoft University is operated by VoIP Training LLC. For a small fee, you can do an online course, and become a BroadSoft Certified Platform Administrator, BCPA. The online course lets you download some of the key BroadSoft documentation. This means that an outside party who's interested in BroadSoft can now access actual useful information without signing a BroadSoft support agreement.
I doubt that this release of info is part of any grand strategy by BroadSoft. Most of the people taking the course likely already have BroadSoft support agreements, and thus have the documentation already. But the BroadSoft University site is separate from BroadSoft's own site, requires a separate login, and separate payment. The technical information on the product is getting out there where technicians can get to it and use it more easily. This is a small step, but an encouraging one for anyone trying to learn about or troubleshoot BroadSoft platforms.