But the ATA says enforcing the Registry code (which lists fixed-line and mobile phones) on foreign companies will be near impossible, despite their legal inclusion.

"It will be difficult to enforce the Australian legislation offshore," the ATA said in a statement. "The 2.8 billion [foreign telemarketing] calls per year, despite being legally bound by the Register, will be almost impossible to enforce."

ATA executive director Michael Meredith said for the Register to be effective, it must focus on stemming overseas telemarketing.

"With Australians receiving such a large volume of unwanted calls from overseas telemarketers, I can only emphasize the importance of using the Register to capture data that could potentially help develop future strategies to reduce the number of overseas centers calling Australia," Meredith said.

The ATA says the Register will lose effectiveness due to the number of telemarketers given exemptions, including registered political parties, independent members of parliament and candidates, charities, religious organizations, educational institutions (for calls made to students, alumni or alumnae) and government bodies.