Yes, one modem will do. But every time you change the telephone cable (RJ11) to the DSL port of modem for new connection, you need to log in to the modem using 192.168.1.1 and change the ID and password. You will 20 % off on both rental and usage if both connections are in your father's name. If you do not want to make calls from your land line, you can choose UL 750 Combo and Home Combo 500 or Home Combo 600. The later has free calls 175 and 250 respectively.
