SSinger1 has some good observations. I think this is where the famous quote is most applicable:
"The difference between a smart mand and a wise man: A smart man learns from his mistakes. A wise man learns from other's". So allow me to provide you with the chance to become a wise man........
I was facing the same question after we purchased our system from a distant vendor. Like normal, the management sat on the contracts; lost the contracts and pretended that they never received them. My vendor didn't go a good job during the initial installation so, I didn't push the management to sign nor did I submit replacement contracts for them to sign.
What I've seen suggests that most contracts are comprised of three parts:
The first being the monthly PM stuff like checking the UPS, checking on the backup routines, checking on the T1 circuits, etc. Often times, it includes software upgrades and phone instrument repair services. In my opinion, the need for this service is fully dependent on your confidence in the contractor providing the service. If they're fly-by-night, there's a high probability that they'll bill you every month but forget to do the work. Get a repetuable company and it is all worth the cost. In my case, I think I might have made a mistake. Nortel is suppose to build good stuff. In my opinion, I think the 3903 phones aren't as robust as they should be and I think I'm suffering too many failures which costs me. Out of the box we had 25% failures. Since then I'm getting about a 10% per year failure rate. With a contract, I could have pushed the problems to the vendor and Nortel. Since we didn't sign, we're paying for the repairs.
Second part of the contract is the off-site management. This is where they install a device to monitor your system and call you when something goes wrong. They're supposed to made all the arrangements for repair with the LEC and/or send someone to repair your PBX. Note however that they then bill you for sending a tech to your site. After installation, my vendor was suppose to provide 1 year of monitoring. This is one of the places where my vendor didn't perform to my expectations. A few times, I had to call them and tell them that my phone system was down.
And last part of the contract, is the MAC (move, adds, changes). My vendor is 75 miles away. In my case, this is a hard sell given the minimum charge rates and the fact that I pay for windsheild time. A while back, I paid for some MAC time and in the end I was disappointed. They billed me for 100% but only performed about 80% of the work and I had to do the rest. I think this is why I didn't push the contract with my management.
I'm not saying that I know any more than the contractor or doing any better but then again, the phones keep ringing and it's given me a chance to learn. You could look at it another way...I'm salary, my time is all ready paid for whereas the contractor gets paid out of your O&M budget.
Long story short....It's all dependent on how confident you are with your skills, the skills of you contractor and how much money you've got in your budget.
mike