I only use external modems for the extra protection from lightening and any other surges on the telephone line.
This time of year we have a lot of thunderstorms and all the telephone lines are above ground so they can pick up a significant voltage from a near miss from a lightening bolt. Actually the only damage is to 3 flat pack resisters that connect the main modem circuitry to the telephone line, but I'm too cheap to buy a resistor kit for $100 when I could buy a new modem for $29. The small flat resistors are only available in a complete kit of common resistance values. My DSL service will only cost me $29 a month, now that it is available in my area. My dial up service was almost that much from the same company .

With the DSL service they install heavy duty lightening arrestor at the main junction box on the side of the house. Any surge is shunted to ground and the relay resets once the voltage is gone.