This is a normal phone jack. Modems usually have 2 jacks like this, one to connect to the phone line, and the other to connect to a telephone to use when the modem isn't in use.
A modular 4-wire jack and/or connector typically used with copper cable having two twisted pairs usually unshielded twisted pair Category 3 or Category 5. Used for telephony, 10Base-T and 100Base-Tx Ethernet LANs, and Token Ring LANs.
The typical four or 6 wire connector used to connect telephone equipment.