The first problem arises from the meeting of the minds. Between the installing company and the customer, there needs to be clarity in what to expect and how features and applications will work before cutover and not afterwards. The provider must set the appropriate expectations and this requires more work than some are willing to provide. As customers you can ask, "Does the system provide unified messaging?" Most vendors will say yes. but unless you clarify what and how, expect very little in the way of true unified messaging (UM) capabilities. UM consists of numerous definitions by vendor so it would be wise to determine beforehand what UM means to your company and do more than just take the vendor's pulse.
The second issue is lack of planning. For every hour of planning not spent up front, expect to spend ten hours or more on the backend. Whether your network was assessed and properly can make a huge impact on cutover. The telltale signs will be the number of hours spent troubleshooting issues and how long it takes to resolve each issue after cutover has begun.
The third problem is lousy installation practices. hese come in the form of ugly installations -- meaning if the housekeeping is poor, so is everything else. An ugly install won't look professional and if the install doesn't look professional, then you can expect problems. Included in this is the lack of wire management or cable termination practices. Power and circuit protection are missing and UPSes are not right sized if at all, or the wrong types of UPSes are used. Wire gauge makes a difference too especially in PoE. There may be too many routers or brands of gear impacting voice quality. Then, the gear may lack adequate management and reporting capabilities or the staff may not know how to interpret what's being reported.