[cisco-voip] recommendations for handing off sip trunks as pri for legacy

[chris]

We have recently acquired a new location which has a legacy analog pbx
and a carrier who was providing service as a PRI.

The carrier has now stated they will no longer be supporting PRI and
are recommending a switch to SIP trunks, which would be fine if they
provided adtran or similar to handle the conversion.

The carrier says they do not get involved in this situation and its up
to the customer, so we are left to fend for ourselves :)

This loction already has a 2851 with ipvoice, and I was thinking I
should be able to do everything I need with that?

Googling seems to turn up lots of configs which simply terminate local
calls to a physically connected PRI, when in actuality I want to do
the inverse and use the cisco to hand off a traditional PRI to the
analog system.

Anyone gone down this path before? Have any config examples of what I
described that I can reference?

Also what type of linecard would I need to handoff the PRI? Does it
need to be a MFT T1 card or plain t1 dsu/csu?

thanks in advance
chris