The Morrison government's reheated attempt to ban mobile phones in immigration detention has been effectively sunk.
Independent senator Jacqui Lambie on Friday announced she would vote against the bill after consulting the public.
The influential cross bench senator said she had initially been on the fence.
"I thought there were some good points and bad points to the bill," Senator Lambie said.
"I asked the public what they thought and I got over 100,000 people write back to me and 96 per cent of them wanted me to vote no.
"The sheer numbers for and against aren't the only thing I took into consideration when I made this decision to oppose the phone ban bill. But it was an important one."
The government claims mobiles present an unacceptable risk and have been used to facilitate escapes, smuggle drugs and organise criminal activity.
Labor and the Greens oppose the bill and Centre Alliance MP Rebekha Sharkie voted against it in the lower house.
It's currently before the Senate, where the government needs three crossbenchers to support it.