Four SIP Trunking Operational Challenges:
1 - Interop with PBX or Session Border Controller (SBC), using a SBC would be preferable for many reasons. Best case an Interop is equal to a TDM PRI turn up, but in most cases interop is much longer, and much more difficult to troubleshoot with jitter, packet loss and latency, QOS all an issue.
2 - Bandwidth - You can scale SIP Trunks dynamically, but you still need the required bandwidth for the concurrent calls, and using Public IP can lead to QOS challenges and a denial of service attack, for enterprise grade SIP it would best be delivered via Private IP (MPLS).
3 - Costs - Some carriers, like Verizon, charge a per SIP trunk fee, meaning you would be paying for IP Bandwidth, SIP trunks, and LD or Toll Free per minute costs. You could end up paying more than you would with a tradiitional configuration of a PRI card, local or LD PRI, and per minute rates.
4 - Switches and Network - Some carriers have active-active switches, some have active-standby and some only have one switch. In some business continuity scenario's if the main switch is lost SIP trunks may need to be rebooted to be active on the standby (backup) switch. Best to know the location of the switch your getting the SIP trunk from, the network connection from your sites to the switch, and the carriers Business Continuity and Disaster Recovery Plan.
I'm a big fan of SIP Trunking when and where appropriate, but unlike a trunk or PRI connected to a Class 5 switch at a central office, which is built for disaster recovery and business continuity, and to a lesser extent a regional Class 4 switch, a SIP trunking softswitch could be anywhere, and many carriers only have one switch.
No IT/Telecom manager wants to get a call from a C-Level executive asking why his or her calls are garbled, from a call center manager that they can't take sales or customer service calls because their toll free is down (due to carrier X's switch getting whacked by a tornado or hurricane and you're 2000 miles away from that catastrophe), or from finance asking why costs have increased with the new technology upgrade to SIP (which was supposed to lower costs).
It's easy to say SIP is the best solution, and the only viable option in 2011 and beyond, but being on the front lines of TDM, VoIP and SIP for 10+ years I think it's wise to hope for the best and plan for the worst, and consider getting an experienced telecom consultant or telecom agent to help you.
About Vocio: Vocio is an industry leading Telecom Consulting Firm with a ten year track record of providing legacy and next generation voice and data carrier services consulting. We also provide Telecom Expense Management, Mobile Device Management, & Telecom Auditing. Our clients range from small enteprises to the Fortune 500. We can be reached at 888-200-8647.