Here's an interesting Skype solution that lets you integrate your PBX with Skype/SkypeIn,SkypeOut so you can use Skype for your trunk lines to save money. I actually found this on TMC's VoIP forums.

Here's a description of the product carried by Spintronics:
. Design Power boost to ensure enough voltage and current to drive your PBX and Ring
. Other USB TeleBOX RJ-11 cannot drive most PBX because their volrage and current from small-power PC USB port
. Connect to your company PBX and take advantage of Skype/SkypeIn/Skypeout services
. Employee can use SkypeOut for low-rate call
. Customers can use regular phone and call SkypeIn number for local call service
. Skyper can call customer service for free
. Use Skype account: spin_callcenter ext.21 to feel it and ask the details
. People who live in USA can use regular phone to call 415-508-3685 ext.21 to feel it and ask the details

Specifications
. Protocol: Skype compatible
. IP management: DHCP/Static IP
. Four port RJ-11 FXS
. Connect to PSTN phone(wire or wireless) or PBX(Digital or Analog)
. Ring (ITU standard-like)
. Dial Tone
. One RJ-45 10/100 Fast Ethernet (LAN)
. Web-base Administration
. Dimension: 19" 2U standard chassis
. Input power: 110/220 VAC, 5A

On their website I see a 1-port, 2-port, and even 4-port solution called CallFree 400. I wonder how they interface with the PBX? I'd be curious to know whose TDM hardware they are using to interface with the PBX.

The website seems a tad "simple" for me and it loaded slowly, so I'm not sure if this a one-man shop building these Skype-to-PBX gateways, or a reputable shop, so just be careful.