The Department of Communications has revealed that 17,500 of its satellite users have no 3G mobile coverage.
In response to a Senate Estimates question on notice, the department said it estimates there are 201,000 fixed voice services being provided by Telstra in the Sky Muster satellite footprint.
Of those 201,000 fixed voice services, 181,000 or 90 percent are provided via Telstra's copper network; 14,000 or 7 percent are provided using radio technology; 4,500 or 2 percent use Telstra's terrestrial wireless networks; 1,000 or 1 percent use Telstra's Universal Service Obligation (USO) satellite service; and 500 use Telstra's Velocity fibre network.
In regards to mobile coverage, of those 201,000 fixed voice services, 154,500 or 77 percent are located within at least one carrier's 3G coverage; 29,000 or 14 percent are located outside any 3G coverage, but within at least one carrier's 3G external antenna coverage, 17,500 or 9 percent have no 3G coverage.
"These numbers are based on a desktop assessment of mobile coverage, and do not take into account factors that could contribute to poor mobile coverage like obstructions from buildings, trees, and hills," the department added.
"Field testing would be required to confirm these numbers."
Last week, Communications Minister Mitch Fifield said the Australian government has decided that the Universal Service Guarantee (USG) will be best delivered by the existing USO contract with Telstra for voice services.
"Regional consumers outside the NBN fixed-line footprint have expressed deep concerns about the possibility of losing access to their existing copper line services, particularly in the absence of multiple proven and reliable alternatives," the report said.
According to the department, there are "clear concerns" in relation to the reliability, quality, and redundancy of alternative voice services, with a "general distrust" for satellite.
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Removing the Telstra copper continuity obligation from the USG would mean 235,000 ADSL customers would need to migrate onto the NBN's regional offerings, the department said.
In its Development of the Universal Service Guarantee: Summary Report [PDF], the Department of Communications proposed further work across trialling alternative voice solutions and engaging with Telstra and NBN on alternative voice solutions.
The department said any cost savings attained by delivering voice services in regional areas using new technologies would be negated by putting more pressure on NBN's satellite.
"We have listened to regional Australians and heard the importance of maintaining existing copper and wireless networks for fixed voice calls in NBN's fixed-wireless and satellite areas," Fifield said.
"We will not make changes that affect regional Australians until there are robust and proven alternatives to the existing USO."
The 2018 Regional Telecommunications Review [PDF] was also tabled last week, recommending that the government not make changes to the USO until there is a voice solution for those living in regional areas served by NBN's satellite.
The independent review committee likewise said industry should be involved in coming up with "new and innovative solutions for providing voice services".
"The services covered by the Universal Service Guarantee need to meet the legitimate needs of regional, rural, and remote Australians," the committee wrote.
"For many ... Telstra's fixed voice service is the only connection to the outside world if their internet service is not working. This is particularly the case for Sky Muster customers that do not have mobile coverage. These people want to retain their landline services as they cannot rely on the Sky Muster satellites for voice services, which they were not designed to deliver.
"The committee supports a measured and cautious approach to any changes to the Universal Service Obligation arrangements."
Updated 16.10 AEDT, December 12, 2018: Article previously stated the answer was from NBN rather than the Department of Communications; this has been corrected.