Millions of mobile users in the UK have yet to receive the government's text message alert about coronavirus.
The SMS - telling people to stay at home - began being sent early on Tuesday morning.
But Vodafone has confirmed it only expects to complete the process later this Wednesday.
One expert said that it highlights the shortcoming of not having a system that can flash a message to people's phones simultaneously.
Vodafone is the only one of the UK's big four networks that has not finished the task.
Three told the BBC it had delivered the text to a total of 10.7 million phone numbers by early afternoon on Tuesday.
EE said it had also completed the process yesterday, having transmitted about 1,000 messages a second.
O2 also confirmed that all its subscribers had been sent the post.
Vodafone said it was still sending out texts in batches of millions an hour, but needed until some time Wednesday afternoon to finish the process.
"We have sent out the majority of messages and aim to complete the process soon," said a spokesman.
"We did pause sending them out overnight so as not to disturb people whilst they were sleeping."
The firm temporarily halted the process at 20:00GMT to avoid waking phone-owning children. It needs to send a total of about 18 million texts.
The government's full message read:
GOV.UK CORONAVIRUS ALERT. New rules in force now: you must stay at home. More info and exemptions at gov.uk/coronavirus Stay at home. Protect the NHS. Save lives.
The Cabinet Office ran trials in 2014 for a system which bypassed the mobile networks and sent messages directly to people's phones, but it was never developed. One operator has suggested that it was considered too expensive.
Other countries, however, have financed such technology and used it during the current crisis.
Ben Wood is a telecoms consultant who helped launch Vodafone's text message service in the early 1990s, when he used to work for the firm.
"SMS is attractive to the government because it's a lowest common denominator messaging solution - it can reach many of the most vulnerable people in society who only have a basic feature phone.
"But it's not really fit for purpose because the texts have to be sent out sequentially, meaning people get them at different times.
"What we really need is a wireless emergency system like they have in the US, which would allow the government to get the same message to everyone at once - not just for coronavirus, but for things like major weather disruption, that pose a danger to life."
One benefit of such a system is that the messages can be geographically targeted, meaning they can be limited to a specific zone in a city to - for example - help find a missing child.
However, not all mobile phones or networks support the facility.