I've said for years that there should be a turnkey "box" that you can simply plug into a PBX and turn it into a PBX with VoIP trunks (or IP Centrex if you prefer.) The box would "simulate" analog or T1/E1 trunk lines so the PBX simply sees the box as the Central Office (CO) providing trunk lines. Then an Ethernet connection on the box would hook up to some sort of broadband IP connection. All voice traffic would get packetized and sent across this IP connection and terminating on an ITSP's (Intenet Telephony Service Provider) network, such as Level3 or Global Crossing. Maybe the box would have expansion ports for a 2nd or 3rd T1/E1. Hey, maybe I should be a VoIP product designer!
In fact, TMC Labs wrote about Net2Phone's MAX T1/E1 product in 2002 which in fact did just exactly that. Net2Phone MAX T1/E1 Review It earned a B+ overall rating in the the review we wrote.
The problem is that it was pretty pricey - nearly $5 grand and as I don't think it did well in the market. It may not even be offered any more. (I checked their website, and I only see a 2-port, 4-port, and 8-port analog versions though.)
A "box" should cost maybe $1500-$2000 (subsidized by 1-4 year contract with an ITSP if need be), support at least one T1/E1 (24 channels), have expansion ports for adding analog or more T1/E1 ports, and hopefully use good sounding codecs and industry standards, such as SIP.
Just think of the market potential for such a box. There are literally hundreds of thousands if not millions of traditional PBXs still on the market. Many companies are in long term leases, or simply cannot afford to throw out their existing PBX in favor of an IP-PBX which can cost anywhere from $15,000 - $150,000 or more. But they want to jump on the VoIP bandwagon somehow. Well, they can on the trunk side at least and get all the cost savings offered by internet telephony service providers, which offer cheaper per minute rates than traditional carrier trunk lines.
So while they won't get the benefits of "station side" VoIP using some cool newfandangled VoIP phone, they still get the cost savings on the "trunk side".
Then, hey, with all those cost savings maybe they can save up for that new IP-PBX they've got their eye on.
UPDATE! A source has told me that later this year a "major ITSP" will introduce an IP adapter that will plug into the back of a legacy PBX.