Like hundreds of large corporations, Mobil Oil Corp. relies increasingly on facsimile machines to zip documents from one branch office to another.

But when the petroleum giant recently received several mysterious invoices, which appeared to bill the company for $980 so that it could be listed in an international telefax directory, Mobil packed them up and sent them off the old-fashioned way _ through the mail _ to the fraud unit of the U.S. Post Service.

During the past month, companies across America have sent the post office copies of similar invoices that ask them to send money to cover the cost of inserting their fax numbers into a directory published by TTS Communications Est.

Several fax-industry experts and postal officials say they have never heard of the company or the book it is selling, Directory of U.S.A. Telefax Subscribers. The company does not have a phone number listed in Zurich, Switzerland, where the invoices say it has a post office box.

"The last time we counted, 72 complaints have come in about this company," said a representative of the postal service's international fraud investigation unit in Washington, who asked not to be identified. "And we've gotten a lot more since then. They're coming in every day."

The explosion of the fax business to an estimated 1.5 million users by the end of this year has opened the way for this new twist on an old scam.

Post offices get "hundreds of thousands of complaints a year" about companies that send out bills for various services that were never ordered, said Thomas Ziebarth, an attorney with the postal service.

"Everyone has a fax machine," said Patrick Evans, president of Evcor Systems, a facsimile network service based in Lisle, Ill. "There's going to be a whole bunch of people who will send stuff like this out."

Legitimate directories of fax numbers are published. Daniel Dummett, president of Dial-a-Fax Corp. in Deerfield Beach, Fla., says that he and others who publish legitimate directories don't charge companies to be listed. His company publishes a directory of 130,000 fax numbers.

"Why someone would pay $980 for a listing, I don't know," said Dummett, who says he makes his money by selling the book for $69 a copy to businesses and through advertisements. "We don't charge people to have their number in there."

Fax machine owners are vulnerable to scams because in many cases they are large corporations that aren't as likely to question bills. And, it is simple to get a list of companies' addresses and fax numbers. As fast as fax machines are becoming standard fixtures in American offices, legitimate directories of their numbers are proliferating as well.

The fax scams find unwary victims, including the postal service itself. "Some of our branches, the bigger ones with telex machines, have paid for listings in these directories," said Ziebarth, who is on the service's consumer protection board. "It's ironic, but those things do happen."

Evans' business reflects the popularity of fax machines, which many companies are using to partially replace overnight delivery services. Evans handles direct marketing campaigns via facsimile machines and sells fax machines and products.

And, last week, he received a bill for the fax directory from TTS Communications. Evans said he filed the bill in his paper shredder.

The postal service has received complaints about similar solicitations involving several other foreign companies, according to the fraud-unit employee. "They come from Canada, West Germany, the Netherlands, London, Holland _ you name it," she said.