Probably because you have WiFi/WiFi calling enabled. Try disabling it!

WiFi calling is disabled, just checked.

Yes, that's what I think. Not good enough though if you ask me, network providers shouldn't rely on customers using their WiFi network to free up load. They should be able to offer the service we pay for IMO.

Hi ​@Slowmilton 

I am so sorry that you are experiencing network issues. 

Please use the following link to check the coverage on your area:
 https://www.idmobile.co.uk/help-and-advice/coverage

Or try to reinsert your SIM or try your SIM in another phone. This will determine whether the issue is with handset its SIM card itself.

If the issue persists, please try the following steps for iPhone:
iPhone (iOS) – Reset Network Settings
Go to Settings → General.
Tap Transfer or Reset iPhone.
Tap Reset.
Select Reset Network Settings.
Confirm the reset when prompted.
Once complete, return to the home screen and restart your iPhone.
 Samsung – Network Settings
From the Home screen, swipe up to access Apps.
Go to Settings → Connections → Mobile Networks.
Tap Network Opera that yours.
Turn Select Automatically on or off.

Network settings for Samsung: 
Go to Settings
Tap General management
Select Reset
Choose Reset network settings
Confirm by tapping Reset settings

Thank you,

Zandile 

It's just a network issue, the service is working haphazardly, I have 4 and 5G but the service is super slow. Not good enough when my 02 work phone is working fine.

Hi ​@Slowmilton 

Thank you for getting in touch with us, and I’m very sorry to hear about the slow network performance you’re experiencing.

Please send us a private message so we can securely access your account and escalate this to our technical team. We will investigate the issue further, especially as you mentioned that you have 4G and 5G but are still experiencing poor service.

Thank you,

Zandile