Transparent bridging

[riwer]

Hello everybody!!
Company that I work for have 4 “remote” offices that have no direct connection between them but they are all connected to internet by same ISP …. This is getting complicated so hire is the sort of diagram

Remote office network --> (my) NAT Gateway -->
ISP Network --> NAT gateway --> Internet

This is same for all 4 remote offices.
Now, ISP network has security issues (no security) so my gateways can see each other through ISP network. Also I have in every office Cisco router (3640 / 3745) which I use for VoIP with at least two free Ethernet ports.
Now my question is: how can I use router to connect remote offices by ISP network but to make ISP net completely transparent.
Boy do I suck at explanations.

Thanx

 

[lambent]

try running site-to-site VPN then

http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a0080093dc8.shtml

http://www.cisco.com/en/US/tech/tk583/tk372/technologies_configuration_example09186a0080094bff.shtml