Despite sharing the nomenclature of "modem", cellular connections are quite dissimilar from analog PSTN/POTS dial-up connections (e.g. V.92).
Your Opengear cellular modem connects using a packet-based IP interface that has more in common with a Wi-Fi or Ethernet connection. This connection differs from a dial-up connection in many ways, including:
You do not dial-in to a cellular modem using the SIM phone number. There did exist an interim GSM technology known as CSD (circuit-switched dial) that facilitated this using specialized equipment hosted by the cellular carrier or telco to convert between analog and digital signals – however CSD has been largely phased out.
You don't need a second cellular modem or other specialized device to connect to the Opengear's cellular modem. Access is generally via the Internet or private carrier network using standard IP networking.
When configuring the cellular connection, the dial-out number is not a "real" number. It is an instruction to the cellular modem to activate a SIM profile and can generally be left at its default.
For 3G cellular modems, the PPP connection is simulated and established between the operating system and the cellular modem – it is not a PPP connection to the cellular carrier. For enhanced performance, 4G cellular modems use "Direct IP" rather than PPP interface, which from a operating system perspective appears more like an Ethernet interface.
It is important secure your cellular IP connection as you would an IP broadband connection, refer to this article for best practices.