I’m a little confused by your description but I’ll try. They have servers in the US they need to access. They can dial up to them, (Is this a direct connection to the network their servers are on?), but they can’t reach them from your internal net. Is the problem caused by their firewall?
If they are making a direct connection to a modem server then you need a different approach unless you can provide them with analog lines for dialup. I would hesitate to give them this kind of access if they are also on your internal network.
Do they have a VPN server like your PIX? The next two options depend on this.
How much do you trust them? If you trust them you could set up a VPN between your PIX and their VPN server and route their traffic through it.
I would lean more to setting up a separate subnet for them, (give this net free access to the internet but firewall it from the rest of your internal net), and let them VPN to their own network.
Can they reach notes using some kind of web server? If so, this would allow transparent access through your squid and PIX as well as their firewall.
Hope this helps.