MCI Communications Corp. getting into your laundry detergent? GTE/Sprint popping up in your Shredded Wheat?

Not quite, but close. The two long-distance telephone competitors are locked in a boxtop battle to sign up new customers. MCI has teamed up with Procter & Gamble Co., and Sprint has enlisted Nabisco Brands Inc. in the new marketing ventures.

AT&T Communications, the long-distance arm of American Telephone & Telegraph Co., also began testing the waters about a year ago with food products such as Maxwell House Instant Coffee. The company has been offering gift certificates for long-distance service for five years.

MCI gave Procter & Gamble the go-ahead last month to insert 30 million coupons into 150 newspapers inviting consumers to sign up for MCI long-distance service. Procter & Gamble also is promoting the idea with television commercials.

Customers who call a toll-free number to arrange for service automatically get 20 free minutes of calling time, according to MCI spokesman Gary Tobin. Customers may then send three proof-of-purchase seals from Cheer, Era or Cascade detergents to Procter & Gamble in exchange for 40 additional minutes of calling credit. That comes to about $10.70 worth of service.

The offer expires Dec. 15, and only new MCI customers are eligible.

"Procter & Gamble is a good marketing company, and makes good products," said Tobin. "It was natural for us to align ourselves with companies that have respect in their markets and have a good reputation for customer service." He said it was too early to tell whether the new marketing strategy is attracting large numbers of new customers.

Meanwhile, Sprint is going all out in its joint promotional venture with Nabisco, which features coupons on boxes of Shredded Wheat breakfast cereal.

"Sprint is a transparent product with no retail outlets," said Maureen Kleier, Sprint's promotion manager. "Through the use of a major package goods company, we thought we could gain some higher visibility."

Sprint is giving away $5 of calling time to new and current Sprint customers until March 31. Customers must call a toll-free number and send in two coupons from Shredded Wheat cereal boxes to receive the credit, which will appear on their next bill. Ten million boxes of Shredded Wheat have the coupons on them. They began appearing on grocery shelves in mid-September.

The offer has attracted 200 people a day, most of them first-time customers, Kleier said. The promotion is being supplemented by full-page ads in women's magazines such as Good Housekeeping and Redbook.

Sprint selected Nabisco because it is a major company with an an extensive line of packaged goods, which could be used if Sprint decides to expand the program, Kleier said. Shredded Wheat also is "the No. 1 adult cereal," she said.

AT&T Communications also believes coupons have a lot of potential. "We're very optimistic about the future of the certificates," said Donna Gemme Brooten, the company's marketing staff manager.

The company teamed up last April with General Foods Corp. to offer a $5 long-distance calling certificate in exchange for nine proofs of purchase from Maxwell House Instant Coffee.

AT&T also worked out an agreement last spring with General Electric Corp.'s former housewares subsidiary. In exchange for buying a small appliance, customers could obtain a certificate entitling them to $7 worth of phone calls. AT&T also has offered free service certificates in the past for proof-of-purchase coupons from Tropicana Orange Juice as well, she noted.