One of the most important decisions that must be made early on in an implementation is what products to utilize.
Traditional PBX vendors have been able to dictate which hard and soft phones you have to use and thus are able to control feature set.
Part of the bugaboo of a standards based solution is that all products do not support all of the standards. You can expect that all products will not have equal functionality. Even if a vendor does support the standards you are looking for, they may have errors in their code or they may have interpreted the standards differently than other vendors.
Testing is critical to the process. Make sure the equipment you select will match the requirements you have.
So, what are your requirements? Low cost, certain features?