Short answer: not always. You can keep your primary SIM on for calls or SMS, but you should prevent it from using roaming data if you want to avoid charges.
Recommended setup
- Use the eSIM label for your 4S profile as mobile data.
- Turn on data roaming for that same eSIM line if required.
- Turn off data roaming for your primary SIM.
- Disable mobile data switching or automatic data switching if your phone has that option.
Your primary carrier may charge for calls, SMS, voicemail, or roaming services. Check with your primary carrier before travel.
Need help?
If you are not sure which SIM should use mobile data, contact 4S support through support@4s.net, a Zendesk support ticket, or Zendesk live chat. Include screenshots of your SIM/eSIM list and mobile data selection screen.