"How are you connecting to the phone network? If it is a dial up connection, like the windows-based VPNs then there is a setting in the IP Protocol for the connection that essentially tells Windows to route all traffic through the dial up connection. Perhaps, you could simply un-check the box, called ""Use default gateway on remote network."" This is found in the Advanced settings of the IPv4 protocol under the Dial Up Connection.
In the end though, the box I mentioned above is just manipulating your routing table, so @Chris Thompson's answer still applies. Either manually adjust the routing table so all machines are accessible at the same time (assuming it is possible and there are not IP conflicts on the two networks) or possibly try the setting I mentioned.
