MARTIN Lewis' MSE has warned thousands of mobile customers to make a crucial move now.
It comes as Plusnet, part of the BT Group, is permanently closing its mobile arm in weeks.
The firm currently offers broadband and telecom services to over a million UK customers.
But all Plusnet sim cards will stop working once you've received your final bill this month.
Martin Lewis' team at MoneySavingExpert.com (MSE) said: "If you're still using a Plusnet Mobile sim, you need to switch away as soon as possible – particularly if you want to keep the same phone number."
Millions of Android owners could slash 'vampire bills' – how to save money
"Existing Plusnet Mobile users will lose access to their service from their June billing date.
"This will vary by customer, though you should have been given at least 30 days' notice.
"After the switch-off, you'll no longer be able to use your Sim card to make calls (other than emergency 999 calls), send texts or access the internet."
To avoid losing connection, you'll need to switch providers beforehand.
Plusnet has updated its rules and said that even if you fail to switch, you'll still have 40 days to port your existing mobile number to a new network after you've received your final bill.
Affected customers can switch to another network without a fee and it doesn't matter if they're mid-contract.
To switch before you receive your final bill, you'll need to phone Plusnet at 0800 079 1133 and ask for your final bill.
The MSE team said: "Plusnet Mobile uses the EE network signal (also known as piggybacking) and it's offering users exclusive deals to switch across to EE.
"It wouldn't tell us the exact details of these deals, though it says most can get EE Sim-only deals at no extra cost or better value than their current Plusnet Mobile offer.
"However, just because you're offered a deal, it doesn't mean it's the cheapest out there."
Six savings challenges to take in 2023 - how you could save thousands
We've explained how to search for a better deal and take your number with you below.
CUT MOBILE COSTS
Switching contracts is one of the single best ways to save money on your mobile bills.
But if you can't switch mid-contract without facing a penalty, you'd be best to hold off until it's up for renewal.
In the weeks before your contract is up, use comparison sites to familiarise yourself with what deals are available.
It's a known fact that new customers always get the best deals.
Sites like MoneySuperMarket and Uswitch all help you customise your search based on price, data, calls and texts.
This should make it easier to decide whether to renew your contract or move to another provider.
It's also worth investigating social tariffs. These broadband or mobile packages and discounts have been created for people who are receiving certain benefits.
They're often available to those on income support, Universal Credit, or disability allowance.
You can see which providers offer these cheap tariffs by visiting www.ofcom.org.uk/phones-telecoms-and-internet/advice-for-consumers/costs-and-billing/social-tariffs.
PLUSNET IS CLOSING ANOTHER SERVICE TOO
Plusnet is also in the process of axing its landline services for good.
The telecoms firm told The Sun that it will no longer offer home phone services to existing customers once analogue services are switched off.
This means that customers who still want landline phone service in the future will be forced to move to a different telephone company that offers a digital service.
It comes as all copper-based phone lines will be switched off by the end of December 2025.
To replace this redundant service, telecom firms are beginning to issue landline phones which operate digitally.
These use a Voice over Internet Protocol (VoIP) service and work by connecting to the back of your broadband router.
Existing Plusnet customers can continue to use their existing home phone service for now, but will need to make the move to broadband-only or move company for good in the near future.
Plusnet told The Sun that for those customers who would still like a home phone service in future, exclusive access to the very best deals on BT and transition to their digital landline service will become available.
However, customers who wish to ditch their landline service once their contract expires will also get exclusive access to EE mobile deals to help them stay connected.
Currently, any new Plusnet customers can only sign up for broadband-only tariffs on the firm's website.
This is the third service that Plusnet has announced it plans to axe, after it previously shut its TV service and is nearing the completion of axing its mobile service.