Centralized NOC Monitoring Solutions

I am evaluating outsourced NOC solutions, where all of the device monitoring and pro-active response is handled by a centralized NOC (NaaS? :-)). The goal is to reduce the day-to-day overhead of calling carriers to troubleshoot circuit problems, basic network troubleshooting when congestion occurs, pro-active response and notification when critical systems go down during off hours, etc. The focus is network related to start, but could branch out into servers in the future. Any feedback on: companies used, what to look for, and general thoughts on a centralized NOC approach would be appreciated. -- Eric Cables