Would need your expert advise on the subject mentioned, assuming there are 2 location using a dial-up modem to exchange data using a telephone lines. It is a point to point connection, what is the risks of this setup, can an attacker gain access to a network if the telephone number of one location is known by dialing to that number and steal data from that organization? Please advise and thank you in advance.
Modems can be set to auto answer after a set number of rings, so knowing the number of a modem does create some risk, as they're be something connected to the modem too that you could try to access. A lot of IT kit still contain modems; e.g. many MFDs, mainframes although they're normally configured to only dial home to the OEM to report faults.
Here's a pretty good resource to help you understand the risks associated with modems and the mitigations: