Take a look at AGN Networks if you are looking for SIP trunking. What does that mean? Well if you are looking to put in an IP PBX or you already have one be sure to also consider connecting your PBX to the outside world via a SIP trunk so you can leverage the lower costs of termination via VoIP on all your calls.
AGN works with Cisco, Avaya and a bunch of other IP PBX companies. The partners behind AGN have been in the VoIP services business for many years and furthermore one of the partners owns an interconnect business with over 20,000 PBX installs! Based on the above so I am pretty confident their network should be pretty solid. As always, don’t take anyone’s word for it, do your own investigation before you buy but I would certainly check this company out.