I'll title this answer "why not a Cisco UCCX solution"
Cisco phone systems will always cost you more. They go end of life every 5 years. They require a subscription to keep software images current, even if they are
so buggy that your voicemail gets corrupted. You'll need to hire a Cisco VAR to take care of the system who employs at least a CCVP, CCNP, or CCIE.
The VAR may end up hiring a telephone man to come out and do simple things like, well, cross connect a phone line to the system. The VAR may sell you a $5,000 "paging server" because CCVP's and CCNP's aren't trained on FXS/SLT/LS ports that can easily be used with industry standard paging equipment. The VAR will insist that you upgrade all switches and routers to Cisco brand, and they will probably insist on SmartNet contracts for those too. In short, they'll want you to be a full Cisco shop "or else the phones might not work".
There are loads of examples of all I mentioned...run a Google search or just browse the Cisco forum here.
If you have the Cisco certs or employ someone who does, you might be fine. Otherwise, and this is my two cents only...hire a real telephone man who will sell you a real telephone system.
