It is important to note that FlexiFone's service is not a local UK service but a National Roaming SIM in the UK. If you're using a Voice/SMS plan, your FlexiFone number assigned to your SIM is intended to be used as your Additional / Secondary number.
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 or Premium Numbers: Calls and SMS to shortcodes, premium-rate numbers may not be supported or blocked. Freephone numbers (e.g., 0800) will be deducted from your allowance. This is because your SIM technically operates as roaming within the UK.
Verification SMS from certain services like iMessage 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).
VoLTE is currently in Beta and is available on EE, O2, Three but not Vodafone. As this is a Roaming SIM, the Voice Quality will not be as great as what you get from a local UK SIM.
Device-Related Compatibility: While almost all devices (Apple, Samsung, Huawei, etc.) support 5G and VoLTE with FlexiFone SIM, certain devices like Google Pixel do not support VoLTE and 5G due to sophisticated 'Carrier Profile' requirements.
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.
You may setup conditional forwarding from your Primary SIM to FlexiFone eSIM if that is unreachable, so you don't miss any calls.
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, keeping in mind the limitations above.