'Hi, mom, I'll be late. I'm still 31,000 feet over Dallas'

The captain has turned off the seat-belt sign - so sit back, relax, and call up your cousin in Cleveland.

Telephones, once reserved for presidential planes and plush private jets, will soon be part of everyday travel on some commercial airliners. But because of the cost, chances are they won't be used for idle chatter.

''There'll be times when people are flying with the kids and decide to call grandmother, but the majority of calls will be for business,'' predicts John Goeken, president of AirFone Inc., the Washington, D.C., firm that developed the air-to-ground phone system.

Mr. Goeken, who also founded MCI, the private long-distance phone company, says airborne telephones will allow busy executives to talk to clients and take care of other business during hours which otherwise might be spent reading magazines.

Business travelers ''often arrive someplace with just enough time to get to a meeting,'' says Goeken. ''This system will allow them to make important phone calls while still in the air.''

AirFone has received clearance from the Federal Communications Commission (FCC) to experiment with the phones aboard commercial carriers.

''And they're strictly that - experimental,'' says Frank Wright, a spokesman for the FCC's Office of Science and Technology. The FCC is a watchdog when it comes to doling out space on the limited radio frequency spectrum, so AirFone will have to prove itself before permanent status is approved.

To make a call, passengers insert a credit card or a ''cash card'' sold by the airline into a slot on the phone. That releases the cordless receiver, and from that point on it's the same as dialing a standard phone.

When the receiver is put back in the cradle, the card is released and the charges recorded. Calls will be tallied at a flat rate of $7.50 for the first three minutes and $1.25 for each additional minute, regardless of the distance.

If all goes according to schedule, Goeken says phone service will be offered on 40 wide-body jets later this fall, most likely beginning in November. Most planes will start out with four phones, although only two of those will be usable at any one time.

At first, only long-haul flights such as New York to Los Angeles will feature phones. But by mid-1983 AirFone hopes to be aboard about 250 planes, including smaller jets used for short- and medium-distance flights.

Eleven airlines have agreed to take part in the venture, including United, Eastern, and TWA. The airlines like the idea, especially since they'll scoop up 10 to 15 percent of the profit if the system flies. But financial success of the system remains to be proven.

''To a degree, I think we're the victims of a 'me too' mind-set,'' says John Siefert, director of passenger services for Eastern. ''One airline doesn't want the other guy to have something they don't have.''

United, which is installing AirFones on three jets for the start-up this fall , was involved in a 1980 test of an airplane phone system called Sky-tel, developed by Page America. Sky-tel flew on one DC-10 for a year and was considerably more expensive to use than AirFone.

A third company, Aeronautical Radio Inc., was also competing to become the Ma Bell of the skies. But both Page America and Aeronautical Radio now say they've shelved their projects.

AirFone snatched the market lead late last year, when Western Union bought 50 percent interest in the firm and set aside $10 million for development of the system through 1982. Money was what the firm needed most.

One of the stickiest problems the system faces will be ensuring that planes never fly outside the range of a ground receiving station. Calls are beamed from the plane to one of 37 ground stations being set up across the country, which are in turn linked to the long-distance phone network.

It's not yet known whether those ground stations will be enough to cover the entire country, says William Gordon, director of network planning for AirFone. ''There may be some places in the Northwest that will give us some trouble, because of holes in coverage.''

The system automatically sends a call to the station farthest ahead of the plane but still within range of the transmitter. Since the signal won't switch from one station to the next as the plane travels, calls are limited to between 20 and 40 minutes.

However, not everyone is anxious to have phones cutting in on air time traditionally devoted to naps and novels. Those people will be glad to know that the system only allows passengers to call out from the airplane; no one can call in.