The more sensible howto
1. Learn your stuff: SIP is rather complicated and you need to be able to debug it. A quick glance at a pcap of a failed call with Wireshark can turn a week of waiting in vendor hotlines to a 10 minute quick fix.
2. Avoid solutions where you don't get the manual: Those solutions will likely be installed/configured by sales droids with no clue of what they are doing.
3. Plan to swap systems on the fly: 90% of the equipment and software in that are is absolute garbage, 10% kinda work to some degree. There is no correlation of quality and size of the company. So try to keep your systems interoperable and try to have small subsystems.
4. Try to avoid vendor traps: If possible avoid the "unified communications" area. You will be able to replicate most of the functionality with simple soft clients (e.g. Linphone) and voice mail to e-mail.
5. Price does not correlate to quality: There are hugely expensive solutions which simply won't work, but there are cheap solutions which work perfectly fine and reliably. Typically most systems using an Asterisk or Freeswitch server as their core work, or can be made to work with moderate effort, even under highly unusual situations.
6. Avoid "forced certifications": If your vendor demands that only use certified other components/carriers something is fishy. SIP is a complicated protocol, but usually everything just works. Certifications are usually just there to check for that. If your vendor demands certification, it's likely they speak an overly obscure dialect of SIP which will cause problems without end. Good vendors give you free or cheap methods to try out your setup before you finally commit.
If you act sensibly and avoid the traps you will end up with a system which can grow and adapt to the needs of your organisation. If you just buy the "unified" solution from your large "trustworthy partner" you will end up with a pile of toxic waste troubling you even further than web applications built for IE6. (yes 2 of the "avoid at all cost" companies have been mentioned in the article)