If you have run out of mobile data before your monthly allowance resets, you do not have to wait or change your contract. Add a data only eSIM to your phone and you can be back to normal speeds today, while your current SIM keeps your number for calls and texts. When your allowance renews, you simply switch mobile data back. This guide shows how much data you need for the days you have left, and how to set it up.
The CORAL eSIM UK plans come in 7, 15 and 30 day options with 5GB to 50GB of data, and connect to the 3 and O2 networks.
Bolt-on or a data only eSIM: which suits you
When your data runs low, you have three realistic options. None of them changes your phone number.
| Bolt-on from your network | Wait it out on Wi-Fi | Data only eSIM | |
|---|---|---|---|
| Back to normal speed | Straight after buying | Not until your allowance resets | Straight after setup |
| Your contract | Stays the same | Stays the same | Stays the same; only mobile data moves to the eSIM |
| How much you can add | Whatever sizes your network offers | Nothing | 7, 15 or 30 days, 5GB to 50GB |
| What you need | Your network's app or account | A Wi-Fi connection | An eSIM-compatible, unlocked phone |
A bolt-on is the quickest fix if you only need a little extra for a day or two. If there are still a couple of weeks to go, choosing an eSIM plan that matches the days and data you have left saves you from topping up again and again.
How much data you need until your plan renews
Pick in two steps. First, choose a plan long enough to cover the days until your allowance resets. Then choose the data amount based on how much video you watch on your phone each day.
| Days until your allowance resets | Up to 1 hour of video a day | About 2 hours a day | About 4 hours a day |
|---|---|---|---|
| 7 days or fewer | 7 days, 5GB | 7 days, 5GB | 7 days, 10GB |
| 8 to 15 days | 15 days, 5GB | 15 days, 10GB | 15 days, 20GB |
| 16 to 30 days | 30 days, 10GB | 30 days, 20GB | 30 days, 50GB |
If you mostly use messaging, social media and maps rather than video, start from the left-hand column. The data is a total for the whole plan, not a daily limit, so a heavy day can be balanced out by lighter ones.
How we worked out the chart
Netflix says its mobile app, on the Automatic setting, lets you watch about 4 hours per GB of data. We multiplied that rate (roughly 0.25GB an hour) by the number of days, added headroom for messaging and browsing, and rounded up to the next plan size. For example, 2 hours a day for 15 days comes to about 7.5GB, so the 15 day, 10GB plan fits. Higher video quality uses more data per hour; see our guide to Netflix data usage.
How to add a data only eSIM and keep your UK number
Your current SIM stays active for calls and texts. Only mobile data moves to the eSIM.
With a dual SIM phone, your current SIM stays where it is
If your phone supports dual SIM, there is no need to remove or swap your current SIM. You add one eSIM, choose which line handles mobile data, and you are done. As long as your phone is unlocked, you do not need to contact your network either.
According to Apple, iPhone XS, iPhone XS Max, iPhone XR and later support dual SIM with a physical SIM and an eSIM. iPhone 13 and later also support dual SIM with two eSIMs, so you can add one even if your main line is already an eSIM. On an iPhone:
- Buy a UK eSIM plan with the days and data from the chart
- Scan the QR code to install the eSIM (Settings, Mobile Service, Add eSIM)
- In Settings, Mobile Service, tap the new eSIM line and turn on Data Roaming
- Go to Settings, Mobile Service, Mobile Data, and select the new eSIM line
- On the same screen, leave Allow Mobile Data Switching off
Step 3 is needed even at home: CORAL eSIM lines connect to local networks as roaming, so your phone treats the connection as data roaming. Step 5 matters too. If Allow Mobile Data Switching is on, your iPhone may use data from either line depending on coverage, which can pull you back onto the line that has run out.
On Android, look for the SIM or mobile network section in Settings to choose which line handles mobile data. The exact names vary by phone. Before you buy, check that your phone supports eSIM and is unlocked; our guide to checking an unlocked phone explains how.
Living in Australia
The same approach works if you live in Australia. Use the same chart, and choose from the CORAL eSIM Australia plans, which connect to the Optus network.
Things to know before you switch to a data only eSIM
- The days start when the eSIM connects: CORAL eSIM plans begin counting when the line first connects to a network, so set it up on the day you need it
- No mobile data during calls on your main number: with Allow Mobile Data Switching off, mobile data does not work while you are on a call on the other number, according to Apple
- Visual Voicemail and MMS pause on your main number: on iPhone, if data roaming is on for the line you use for mobile data, your other number disables Visual Voicemail and MMS
Switch back to your main SIM when your allowance resets
Once your monthly allowance renews, go to Settings, Mobile Service, Mobile Data and select your main line again. You can then turn off or delete the eSIM line.
If you forget, mobile data will stop when the eSIM's data or days run out, and your renewed allowance will sit unused in the meantime. A quick check on your renewal date avoids that. For more ways to manage data and lines, see our eSIM tips.
