What is the purpose of the phone jacks on surge suppressors?

Some of the surge suppressors incorporate protection circuitry for telephone lines. There are 2 sockets on these products. By plugging a phone line through the sockets, you can minimize the effects of a surge coming into your equipment through the phone line. Fax machines, cordless phones and answering machines are especially sensitive, and computers with internal modems can be completely destroyed by spikes on a phone line.