Internal modems do not use a separate power supply or require a transformer adapter. The physical connection between the modem and the telephone line. This may require an adapter. A digital vs. analog telephone system. Line noise that could interfere with the modem's signals.
Most phones use modular jacks.
Don't forget to take a telephone cord like our Telespool though. However, there are 38 other kinds of phone jacks in use throughout the world, and if you are traveling somewhere that doesn't use the RJ-11 plug you will need a telephone line adapter . This is important because it can sometimes be necessary to dial manually and then activate the modem once a connection has been made. Remember that some foreign-owned hotels will have the telephone systems of their home countries, not of their host countries. A digital interface can eliminate the need for a direct telephone line attachment.