Nine months ago, Carter Jones had no notion that he could make money selling telephones on his own. Now he's proving it.

The 27-year-old entrepreneur quit his job as a Farmers Home Administration loan assistant last August and opened Phones Unlimited, a retail residential telephone store, in a small shop near the oceanfront.

At first, sales were modest, as Jones knew they would be. At that time, a Federal Communications Commission order allowing customers to buy phone equipment - independent of American Telephone & Telegraph Co. - was awaiting Supreme Court review.

Then the Supreme Court declined to hear the case, and the order went into effect. Two months later Jones moved his store into bigger quarters in a shopping center and sales began to climb.

The store sold about 200 phones a week in December, Jones says, twice as many as during the entire month of November. Sales slowed during the rest of the winter, but are rapidly picking up again, he said.

Jones has 150 phone models in stock, and another 50 he can easily order on request. He says his elaborate and expensive decorator phones are the biggest sellers.

(In the Washington area there are two phone stores, the Telephone Warehouse in The Foundry in Georgetown and Telephone Look in the White Flint shopping mall. Telephone Warehouse co-owner James Prettyman said his six-year-old company has 300 phone styles in stock, making it the "biggest in the world." The company also repairs phones and wholesales to 25 other phone shops.)

The Bell System - including Chesapeake & Potomac Telephone Co. - reaction to businessmen like Jones and Prettymen has been to set up its own network of "Phone Centers" where customers can buy telephone devices manufacturered by Western Electric. The FCC order covers all types of equipment except key telephones and PBX (switchboard) equipment, which will be included as of June 1.

Previously, customers could hook extension phones into the official telephone network only if they rented so-called protective devices from C&P or one of its counterparts. The wiring for the devices prevented customers from making or receiving a call on an extension phone.

To receive a call, you had to answer on your home's primary phone, which had to be rented from the phone company, then switch to an extension. Not many phone company customers bothered to buy additional phones under those circumstances.

But now, customers can buy extra phones independently. They must notify the phone company that additional phones are being plugged in and the phones must be plugged into a standard wall jack. Non-Bell System equipment a customer had before last October may be used, but the phone company requires customers to provide details on name and model number.

One of Jones' more elaborate offerings includes his $219 Figure-Phone - a four-function calculator with an eight-digit display, a digital calendar and clock and a touch-tone phone.