iPhone Data Stops During a Call on the Other SIM

If mobile internet disappears only while you call through the other line, investigate dual-SIM routing before replacing the SIM or resetting the phone. The data line and voice line can be different. Whether data continues during that call depends on the switching setting, carrier support and roaming circumstances.

Establish a reproducible boundary

With Wi-Fi off, load a small web page, place a normal call on the other number, and attempt the same page during the call. End the call and repeat. Avoid a large speed test because it obscures a simple before/during/after comparison and can consume metered data. Record which two subscriptions were involved.

Apple explains that disabling Allow Cellular Data Switching can prevent cellular data during a call made on a different voice line. Its documentation also limits the switching behavior during data roaming. Therefore, a domestic setting that worked previously is not a promise that the same arrangement will work abroad.

Choose a controlled remedy

In Cellular settings, identify the designated data line. Check the contact's chosen calling line separately. Do not infer either setting from which carrier name is most prominent in the status bar.
Inspect Allow Cellular Data Switching. Before enabling it, establish whether the calling line has an affordable data entitlement. Turning a routing option on is not permission from that carrier to use unlimited data.
If both lines have suitable domestic plans, enable switching for one controlled test. Repeat the same normal call and small page load. Change no APN, network mode or other variable at the same time.
If you cannot permit data on the voice line, try making that particular call on the line already selected for data, if it includes voice service. Otherwise use a trusted Wi-Fi connection where appropriate. These are alternative calling arrangements, not a hardware repair.
For roaming or persistent failure, ask the relevant carrier whether the exact plan and device support simultaneous voice/data in this arrangement. Provide the line selections and before/during/after result; do not demand an unlock code for a routing issue.

What each result means

Data returning immediately after the call narrows the fault to the call state. Improvement after switching is enabled supports a routing explanation. Failure on the data line even when idle needs the broader mobile-data workflow. Failure of just one app can be an app/session problem rather than loss of internet for the whole phone.

Model and account differences

Supported SIM combinations vary by regional iPhone. A physical SIM plus eSIM and two active eSIMs are not interchangeable assumptions for every generation. Corporate lines may have restrictions you cannot change yourself. Keep the carrier's support scope separate from the phone's menu labels, which can vary by software and locale.

Boundaries

Do not erase profiles, reset all settings or factory-reset merely because one controlled call interrupts data. Do not enable roaming or switching without understanding which account might be billed. If calls themselves fail, investigate voice service before treating this as an internet-only symptom.