Voice VPN using Cisco

I am trying out Voice VPN using Cisco to connect 3 PABX in 3 countries together.

1. What is the best interface for
Cisco 2600/1700 if the PABX connectios is small (E&M or FXO or FXS).

2. If I use FXS at Cisco, What
card should i prepare for the PABX?
CO line Card? Can the remote site call to the PABX and get connected to the local PABX extension directly. (I am thinking since the PABX is equiped with analog CO line card (not PRI with DID features), it will not switch the call automaticall to the local extension when receiving a remote call from the IP side)

i.e. site’s B phone dial site A’s phone directly.

Site B phone->PABX_B->Cisco_B_FXS
-Cisco_B_Serial -> IP Network ->
Cisco_A_Serial -> Cisco_A_FXS -> PABX_A -> Phone A’s extension.

3. Will the suitation be better using E&M in terms implementation?

4. What is the best way if i have to offer VPN services to different coporates? since there is a chance that company A’s extension may be used by Company B. Will gatekeeper helps to resolve overlapping private dialing plan?

5. Is Ciscoworks Voice Manager
good for managing overlapping dial plan?

Thanks for the advise in anyway.