RAS server using SIP/h323?

Question

We currently have a few modems (using standard POTS lines from the telco) connected to a server 2003 box running RRAS for remote access connections (mainly remote data pushes). We also have a Cisco VoIP system in place in our building utilizing two PRI's. What I would like to do is have any incoming RAS connection go down the PRI, get converted to SIP/h323, and then hit some software listening for SIP/h323 connections and then turn them into RAS connections - basically I'm looking for a server that rolls SIP modems + RRAS into one.

What I have:
Remote site --> PSTN --> Telco POTS line --> Modem --> Server 2003 running RRAS --> LAN

What I want:
Remote Site --> PSTN --> PRI --> Cisco gateway --> SIP --> RRAS server running SIP and emulating a modem --> LAN

Any ideas on software/techniques that can be used to accomplish this? The only thing I found that would be close would be t38modem + null modem software (com0com), but that sounds ugly and sketchy...

Update: Our Cisco gateway is a 2851 and appears to have two PVDM cards... from a show diag:

PVDM Slot 0:
48-channel (G.711) Voice/Fax PVDMII DSP SIMM PVDM daughter card
Hardware Revision : 4.0
Part Number : 73-8540-05
Board Revision : B0
Deviation Number : 0
Fab Version : 04

PVDM Slot 1:
16-channel (G.711) Voice/Fax PVDMII DSP SIMM PVDM daughter card
Hardware Revision : 4.0
Part Number : 73-8538-05
Board Revision : B0
Deviation Number : 0
Fab Version : 04

WIC Slot 0:
VWIC2-2MFT-T1/E1 - 2-Port RJ-48 Multiflex Trunk - T1/E1
Hardware Revision : 0.0
Top Assy. Part Number : 800-22629-05
Board Revision : C0
Deviation Number : 0
Fab Version : 04

WIC Slot 3:
3rd generation - FXS DID Voice daughter card (4 port)
Hardware Revision : 1.0
Top Assy. Part Number : 800-27473-02
Board Revision : A0
Deviation Number : 0
Fab Version : 01

Update: We have a few driving factors to getting such a solution in place.
1) We have many sites all dialing in to our environment, with handfuls of the sites being configured to dial a different number (5 numbers in total). It's our crappy method of load-balancing across the modem lines. We looked at doing a hunt group at the telco level but that was quite expensive, so if we can do it using our existing infrastructure and PRI lines we can not only save the cost of the hunt group but also eliminate the POTS lines.
2) Our current RRAS server keeps locking up the modems for no reason and with no alert, so we just have to check every so often to make sure they're live.
3) Doing this at a software level gives us the ability to scale much easier, faster, and cheaper than using physical modems and lines.

Answer 1

What model is the Cisco? Depending on the Cisco hardware involved it might be easier to just terminate the RRAS there rather than trying to pass it along as SIP to some sort of virtual modem bank. But you didn't supply a model number for the existing hardware.

Updated based on comment providing model information:

I'm not sure I see any good reason to complicate things trying to send the modem calls as VoIP to a virtual modem pool when you could easily terminate the calls on PVDM digital modem hardware in the 2851 and then route it onto the network from there. Either the system running the software modem pool you were initially asking for or the router itself would have to sit on both voice and data networks.

Update: The PVDM modules you have installed are for voice and fax. You still would need the PVDM2-12DM (link to datasheet) you mentioned in the comment to my answer to terminate dial access. Note that the datasheet lists certain channelized PRI interfaces that are supported for terminating modem calls on the PVDM2-xDM DSPs, so you'll also need to check your interface module parts to see if this is a supported configuration. The 2851 has 3 onboard PVDM slots so with only two PVDM voice modules you should be able to add the digital modem PVDM module to the third slot if all other hardware will support the modem termination.

If it weren't for point 2 in your latest update, regarding issues with the current modems, I would have suggested adding FXS interfaces to the 2851 and then using those voice ports as the destination for a dial map for the incoming PRI number you wanted remote clients to access. But if the stand alone modems and RRAS server are giving you trouble that might not be the best option but it could be the most cost effective (to drop POTS line costs and avoid the telco hunt cost since your 2851 would essentially be doing a hunt across the FXS itself).