Short Answer: Yes & No. If it's used primarily for Data-Only then Yes, ofcourse you can 👍! However, if it's for using Voice/SMS ⚠️, then please refer to the long answer below:
It is important to note that FlexiFone's service is not a local UK service but a National Roaming SIM in the UK, primarily intended for Mobile Data use.
Your FlexiFone number is intended to be used as your Additional / Secondary number. Voice/SMS service is just an add-on to our Data-Only plans and you must not use this SIM as a replacement to your Primary mobile number (where you would regularly make or receive Calls/Texts on a daily basis).
However, some customers do use it as their primary line on a second device, such as a Tablet or personal phone, but there may be some limitations, as described below.
Things to Keep in Mind
While using FlexiFone as your Primary SIM is possible, there are some limitations:
No Voicemail or WiFi Calling: These features are not supported due to the nature of the SIM being Multi-Network and not linked to one specific carrier.
No Shortcode, Premium Numbers or Special Numbers: Calls and SMS to shortcodes, premium-rate numbers, and special numbers (NHS 111, etc.) may not be supported or blocked as the called party will view this SIM as a Virtual Number on a Roaming SIM. Freephone numbers (e.g., 0800) will be deducted from your allowance. This is because your SIM technically operates as roaming within the UK.
VoLTE is currently in Beta and is available on EE, O2, Three but not Vodafone. As this is a Roaming SIM, the Voice Quality and overall reliability will not be as great as what you get from a local UK SIM card.
Verification SMS from certain services/apps like iMessage and others may be blocked by some network operators (as our SIM is intended to be used as a Secondary line only and not your Primary SIM).
2FA Authentication OTP Codes from certain banks and other providers may be delayed or blocked in rare cases. For delayed SMSs, this may resolved by switching between networks manually (in order to trigger a new network registration).
Device-Related Compatibility: While almost all devices (Apple, Samsung, Huawei, etc.) support 5G and VoLTE with FlexiFone SIM, certain devices like older Google Pixel 4,5,6,7 do not support VoLTE and all old and newer Pixels do not support 5G due to sophisticated 'Carrier Profile' requirements.
IP Address: You SIM will have a UK - Isle of Man IP address and not a mainland-UK IP as it is Roaming. This means that in rare cases, you may need to use a VPN to access a very few number of apps/services which only allow UK mainland IP addresses.
Best Practices for Using FlexiFone
The ideal way to maximise coverage is to have two active SIMs on your phone with automatic failover switching enabled. For example, if your O2 Primary SIM loses signal or 4G/5G data connectivity, your FlexiFone eSIM automatically fills the gap, ensuring you stay connected at all times.
We are unable to provide any official support relating to Voice/SMS functionality. This due to contractual terms on our Roaming agreements with the UK networks. "A Multi-Network Roaming SIM provider must not encourage users in any way that leads to them using the Multi-Network SIM for day-to-day use and leaving their Primary Mobile network, leading to a loss of revenue for the operator".
You may setup conditional forwarding from your Primary SIM to FlexiFone eSIM if that is unreachable, so you don't miss any calls. You should check this directly with your existing operator, review the settings on your phone, and refer to Forums/Articles on Google.
On iPhones, you can also enable WiFi Calling on your Primary line so it would use Mobile Data from your FlexiFone eSIM when your Primary line has no signal (example below):
The above example screenshot is a Lebara SIM with WiFi calling enabled, and when Lebara's network is not available, calls/SMS can still be made using the Mobile Data connection from the FlexiFone eSIM. This is currently on tested on Vodafone and it's MVNOs and may not work on other UK networks.
Summary
FlexiFone works best as a secondary SIM for data and backup connectivity, but it can function as your primary SIM for personal use or a second device, keeping in mind the limitations above.
Whilst it is technically possible to use FlexiFone or any other Multi-Network SIM as your Primary SIM and nothing stops you from doing so, we are unable to officially market this as a Primary SIM or provide any official support in case if something doesn't work as expected. This is due to regulatory and Roaming restrictions enforced by the big networks (as it's a loss in revenue for them if you don't have a Primary SIM with them and just use a Multi-Network SIM for your day-to-day use.)
You must not rely on our Voice/SMS service for day-to-day use as VoLTE is in Beta, and we provide no guarantees of whether or not your Calls will work reliably on all four UK networks. If you do not need Voice/SMS, we recommend you to use a Data-Only plan as a backup SIM since our Annual Data-Only plans offer a higher Data allowance for the same price as a Voice/SMS plan.
For example, £65/year could either get you a:
5 GB + 100 Mins/SMS per month plan;
or 10 GB Data per month
Choose wisely!