How to Configure Microsoft Lync to Use Cisco 3925 as a PSTN Gateway

We need to replace the current PSTN gateway device (extremely buggy, unreliable) with a certified gateway for Lync. The gateway device we use now has poor support and there is no on-site or after hours support.

When looking for a replacement, I see the Microsoft has certified our Cisco 3925E/K9 as a gateway for Lync. We have a killer Smartnet agreement with 24/7 support and 4 hour onsite, so this seems the perfect fit.

Presently the Cisco device sits outside of our Sonicwall firewall and simply acts as the connection to our Fiber internet link. So at present the only add-on is the SFP module for the fiber connection.

The question is this:

1 What all is required to make the 3925 Lync friendly? Mainly I know we need to at least add some sort of card for the PRI and FXS port. Are other modules recommended/suggested?

2 This Cisco device sits outside the firewall - whereas our current PSTN gateway device (and lync) are inside the network. Is it still possible do this config, or will it create a networking nightmare?

3 Lastly, since we still have a fax line (hence the need for the FXS port) - is it possible to have the Cisco device intercept calls to our fax DID's (we have 18) and route those to the FXS port? That way they don't pass to Lync, which wouldn't be supported.

I had also look at Dialogic as a solution, but given the mission critical nature of our voice service - the Cisco device seemed best. Are there any big tradeoffs going this route? I don't know Cisco IOS, however I do have a contractor who usually helps with config.

The best solution:

If you need PSTN GW for one E1 trunk and a couple of FXS' only, 39xx is a little to big for you.

But, of course, it will serve.

First that you need after recieving signal with PRI or FXS card is to encode the voice and to pack it to IP packets.

Cisco has a special module for this - PVDM. This module is equipped wis DSPs (Digital Signal Processors) to pack voice to IP. DSP calculator http://www.cisco.com/web/applicat/dsprecal/dsp_calc.html will help you to choose required PVDM module.

PSTN GW is not a gateway to public Internrt services, so you can place it inside your FW without any risk of intrusion.

And of course, it is possible to separate incoming in PRI trunk calls, for instance, by destination phone number, and directly translate all the calls with selected B-number to dedicated FXS port.