Three has again apologised to "a small number of customers" without service after it fixed the "vast majority" of issues affecting its network.
Downdetector, which tracks outages, showed more than 12,000 of the telecom firm's customers reported calling and mobile data were not working on Monday.
It is the latest outage to hit Three, after it apologised on both Saturday and Sunday over similar incidents.
The firm said it was "very sorry for the inconvenience this has caused".
"Following an issue with our network that started yesterday, the vast majority of services have been restored," it said in a post on X, formerly Twitter.
"However, a small number of customers may be still experiencing an issue with voice calls.
"Unless your query is urgent, we'd advise against contacting us as wait times are likely to be very long."
Previously, the firm told users that its customer service channels were unavailable, so they should follow its social media feeds for updates.
Three has around 10.5m customers across the UK, according to its website, but it is unclear how many of them are affected by the outages as those who rely on mobile internet may be unable to report the issue.
Reports began over the latest incident at 1000 GMT on 12 February, with frustrated customers taking to social media to voice their concerns - with some even threatening to leave the network altogether.
There have also been reports of the outages affecting some people using smaller providers which use Three's network, as there have been 800 reports from customers of one such firm - Smarty - according to Downdetector.
It is unclear whether customers will be able to claim compensation for the outage, although according to the website of telecoms regulator Ofcom, it "may be appropriate" for providers to offer refunds "while repairs are being made".
The outages come two weeks after the UK's competition watchdog announced it was investigating a proposed merger between Three and Vodafone.
The Competition and Markets Authority (CMA) will look into whether the deal could harm consumers by leading to reduced choice or higher prices - as it would create the UK's biggest mobile network.
The firms said the deal would result in an additional investment of £11bn in the UK.