Different modems can't necessarily talk to each other using their default settings.
Modems use many different settings and protocols.
By sending a modem init string, you can control some of these parameters. They don't typically use any form of error correction or data compression protocols. In many cases, if your modem tries to use either these protocols, your modem will be unable to communicate with the paging terminal's modem. In your init string you should include settings to lock your modem's baud rate to that of your carrier's modem, disable error correction protocols, and disable data compression protocols. No, many different modem manufacturers use their own set of init strings.