Assistance required - Building a Dial-up Network Access Server. Cisco NM-30DM?

Hello! I currently have a project bubbling along in the background to provide multiple (100+) of copper telephone lines to a bunch of people in a field, for "fun". For added "fun", I'd quite like to be able to provide dial up internet, even if it's just for a handful of active users.  But this is a little out of my comfort zone, so I could do with some help!

Cisco NM-30DM Module
Cisco NM-30DM

Update: I've managed to get a working setup with the help of some clever folks from ShadyTel. It's currently in the testing phase - I'll do a full blog post soon.

To provide the copper phone lines I'll be using several Cisco VG224 Voice Gateways with a SIP based back end. Before you say it, yes, I'm aware Dial Up / Faxing / Modems can be problematic over VoIP, but I'm hoping as everything is over a LAN and we have full control over the network (Echo Cancellation, QoS etc) we should be able to keep jitter and latency to a minimum, so I'll accept the risk in the name of experimentation. 

So, I'm looking for a solution to go from SIP to a bunch of modems. I could get a few hardware modems (US Robotics or similar) and plug them into FXS ports, then connect them to a Linux box to handle the connections. But with the best will in the world it'll become cable spaghetti and be a pain to transport / setup / store, so I'm looking for one (or two) boxes that can handle multiple users with minimal fuss.

Enter the Cisco NM-30DM, a Digital Modem Module for Cisco routers. It sounds like exactly what I need, a module that can serve up to 30 Dial Up users via a T1 / PRI circuit ( A technology I know very little about) but what's not clear is going from SIP, to PRI, and into the router containing the NM-30DM. As far as I know it's not possible to go straight from SIP into the NM-30DM, so I'm guessing it'll require 2 routers, one doing SIP to PRI and the other doing PRI to the NM-30DM.

And thats where the issue arises, some sources suggest a 1CT1-CSU or 2CT1-CSU module is required, but others are using a NM-HDV2-2T1/E1, or a NM-1CE1T1-PRI. On top of that, some of these modules aren't readily available in the UK, or cost a small fortune. In the ideal world I'd use two VWIC2-1MFT-T1/E1 cards as I already have them to hand, but it's not clear if that would work. I don't mind spending money but I'd like to get it right first time if possible.

I've got the following bits of hardware already (Listed in case it is useful):

  • VG224 (7x)
  • 2811 (10x)
  • 2821 (2x)
  • 3845 (1x)
  • VWIC2-1MFT-T1/E1 (4x)
  • FXS Cards (6+)
  • NM-32A (1x)
  • HWIC-16A (2x)
  • The usual bunch of WIC-1Ts and Cables

Has anyone done this before? Does anyone have any suggestions? Is there a better way of doing this?

I've drawn a very rough diagram if it helps visualise things -

Rough Network Diagram

Thanks for reading, please get in touch if you have any advice.

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