Hi xirix,
Part 1:
This is really a difficult task. I once had my modem destroyed by a lightening stroke and, while repairing it I noticed the manufacturer uses a 60V switching transistor in series with the line. The differential mode protecting varistor was a 275V. So, the transistor would always break first, and thus "protect the varistor"! And the manufacturer was 3Com/USRobotics, what to tell about the others...
Part 2:
Since the thelephone line is DC powered at 48V to 60V and the ring voltage superimposed is 70 to 90Vrms (128Vpeak) the differential protection should be dimmensioned no lower than 200V.
Part 3:
So for the differential protection you could use a 200V (S10K130) Varistor. But if the switching transistor on your board is a low voltage type, you have to replace it for a high voltage type (Bipolar or FET, NPN/PNP N or P channel depends on your modem card) or the like or there will be no effective protection.
Part 4:
Then there is the common mode protection.
During lightning, high voltages are induced between both line wires and ground. So the discharge path is from the telephone line through the electronics to ground - normally the power mains neutral wire. To protect against common mode, you have to connect the protection from each of the line wires to ground. I would suggest also two 200V Varistor type (S10K130), but away from the computer. Best at the point where the telephone line enters your house.
Part 5:
Varistors are fast and cheap, but they can't stand much energy, so you can also employ a gas arrester between each line wires and ground. I would suggest one Three Electrodes 2026-23 Balanced Trigard from Joslyn -
http://www.jesc.com/products/datasheets/gastubes/
Also far away from computer!
Part 6:
The connection to ground should have a low impedance, so I recommend using a braid, not a wire for that connection.
Have a nice surge protection...
Regards
__JR__