One day in February someone in Occoquan will call up a friend in Woodbridge and their voices will flash in bursts of invisible light down a five-mile strand of glass.

Virginia's first fiber optics telephone transmission system will link the two Prince William County towns, Continental Telephone of Virginia announced yesterday.

Instead of using currents of electricity in a copper wire to carry the voice, the fiber optics system will turn the sound into light and "pipe" it through a glass rod the size of a human hair.

The fiber optics system costs less than conventional phone lines, can carry more calls, and one day could revolutionize the communications industry.

For now, however, it will shunt traffic between the two small towns and give Continental pioneering experience with the new technology.

"The customer will never know the difference," admitted George Edwards, Continental's network design manager, who explained the new system yesterday at a Quantico press conference.

Not so, said H. A. Hirayama, president of the Japanese firm that makes the gear to turn sound into light. During thunderstorms, the customers wont't hear the background crackle created by lightning in conventional phone lines because electrical bursts don't effect the flow of light.

He is the head of NEC America, a subsidiary of Nippon Electric Co., Ltd., a Japanese giant equivalent to a combination of General Electric & RCA.

The cables that carry the light are made by another internationally controlled company, Siecor Optical Cables. Siecor is a joint venture of the Corning Glass Works and Siemans A.G., a big German electrical firm.

The $250,000 Continental Telephone project is a tiny order for the two multibillion corporations, but the fiber optics communications business is only a $50 million a year field so far, said Siecor marketing man Bill Bielowski.

Research and development spending in the field is probably two or three times as much as actual commercial orders, but fiber optics is growing rapidly, he said.

The first fiber optics phone system went into operation only two or three or four years ago, and now there are about 10 experimental systems being set up by independent telephone companies and an equal number being built by companies affiliated with American Telephone and Telegraph Co.

"In this case, the independents are head to head, or maybe a little in front of Ma Bell," said Edwards.

Continental executives stress that the Virginia installation is not considered an experiment. "It's a proven technology. The least expensive way to go," said Edwards.

He estimated that the fiber optic system will cost about 10 percent less than using copper cables and conventional equipment.

Maintenance costs of the system are projected to be about one third those of conventional installations, he added.

The Woodbridge to Occoquan line represents only five miles of the 1,500 miles of new cable that Continental will install next year, but the company is looking for other uses of the system. A line between the communities of Dale City and Triangle will probably be converted in 1981, Edwards said.

The chief advantage of the light system is its ability to carry far more calls than a conventional cable.

The fiber optics link between the two Prince William County towns will have only six fine glass strands. It replaces a conventional cable with 1,818 pairs of copper wires.

While the conventional cable is a couple of inches across, the fiber optics cord is about the diameter of a large lead pencil. That means dozens of fiber cables can be placed in the underground conduit that now carries a single metal cable.

The message carrying capacity of the glass cable can be increased four to six times by using electronic techniques similiar to those that allow an FM raido station to broadcast two stereo music signals simultaneously.

When the day comes that every home has a computer, the lines linking the home terminal to the outside are most likely to be glass than copper because of their greater capacity, Bielowski predicted.