1. Don't buy a brand you've never heard of before. You wouldn't buy a plasma tv from a no name brand and you should trust your business to a no name brand either.
2. Don't buy from a vendor that is very far away from you. A telephone system requires routine service, so a dealer that is far away is going to have trouble proving you with service.
3. Don't buy anything that's self install or buy something your brother in law is going to install. Telephone systems are not plug and play. They take an experienced professional to install, program, and teach you how to use them.
4. Don't just let your tech guy deal with it. Make sure you attend every meeting and tell the sales person exactly what you want. Your IT person is probably not a telecom expert. Make sure everyone is on the same page. Most sales people also appreciate the owner spending time with them during the sales process.
5. Listen to your vendor. They have the experience of hundreds of installations; you probably don't.
6. Don't just shop price or beat the vendor up to save a few more dollars. The installation experience makes all the difference and if you go cheap you won't get the best techs and your vendor won't spend extra time making sure you understand everything.
7. Don't skimp on the hardware, especially the handsets. Telephone systems last 10+ years, so an extra $1000 isn't a lot to spend to make sure your happy for the long run.
Andrew Taub http://www.teleco4.com has been selling and installing digital business telephone systems for over 10 years.