Vodafone will pull the plug on its "3" network at midnight on Friday after a decade-long stint in Australia.
From 12.01 (AEST), customers who remain on 3 will be disconnected, losing their ability to send or receive calls, texts, and data.
Under telco rules, they may also lose their number if they fail to move to another provider within six months.
In a note on its website, Vodafone advised customers to "make a move ASAP".
A Vodafone representative said 97 per cent of 3 customers had already changed providers but would not disclose an exact number.
The closure is set to put more pressure on porting services already under strain after cut-price wholesaler Kogan Mobile went under last week.
The company's more than 100,000 customers were forced to change providers after ispONE, the company through which Kogan bought access to Telstra's network, entered administration, severing its contracts.
The current "strain" on porting services "may result in consumers experiencing some delays", the Australian Communications and Media Authority says on its website.
Vodafone is putting on extra staff "to manage last-minute movers" and will extend porting hours over the weekend, its representative said.
"We are still able to port customers' mobile numbers for a short period after 3 closes tonight."
The Communications Alliance, the industry body responsible for porting once telcos receive a request from new customers, will also extend weekend hours.
Customers have known since early June that 3 would close at the end of August.
It is part of a long-term strategy to create a single brand and network under the Vodafone banner following the company's merger with 3's owner, Hutchison Australia, in 2009.
Hutchison launched 3 in 2003 and it became the first provider to launch a 3G mobile network in Australia.
