FvM is correct in saying TCP/IP communication between modems is difficult unless you can make arrangements with the provider, e.g. modems are usually on private networks behind NAT boxes making inbound TCP connections difficult. In a similar application I had two modems making TCP connections to a server with a static IP address. One modem left data on the server, the other collected it. If timing is critical when the first has sent the data it could send an SMS message to the second modem to make a connection collect it.