Ever laboring to invent new flavors to complement the plain vanilla of basic long-distance telephone service, MCI Communications Corp. of Washington has come up with the Family Reunion plan, one-time conference calling for far-flung relatives.

Customers call an 800 number 24 hours in advance, make a reservation and then connect up as many people to gab with as they like.

US Sprint Communications Co., not to be outdone, is offering its own version, QuickConference. It allows direct dialing and requires no reservation, but limits the parties linked to three.

Growth in the $60-billion-a-year long-distance market is slowing along with the economy as a whole, and carriers are anxious to gin up extra dialing.

So, their experts came up with variations on the three-way calling that the local C&P telephone companies have long offered as a monthly add-on to regular service. MCI plugs its version as fitting in with "rising travel costs and a softer economy."

Of course, nothing is simple in today's competitive long-distance market. A little investigation shows that market leader AT&T has had a similar conference calling service since 1984, but has yet to sell it seriously to consumers. So most are unaware it exists. AT&T officials suggest that for Christmas Day, that's probably just as well -- about 90 million successful and uncompleted calls are expected to clog the company's national network tomorrow and it is not looking to encourage more.