What does Allow Cellular Data Switching do?

Where Allow Cellular Data Switching lives

On iPhone, open Settings, then Mobile Service or Cellular. Apple uses both names. Open Mobile Data or Cellular Data. Under the list of lines there is a switch called Allow Cellular Data Switching. It is not inside the travel plan’s own page. People miss it because the travel plan page has Data Roaming, and this switch sits one level up. On Samsung, open Settings, then Connections, then SIM manager. The control is often Auto data switch, under the SIM you picked for mobile data. Other Android phones put Switch mobile data automatically in the SIM or Network menu. If you cannot see the words, use the Settings search box and try switching, then data. Menu names differ by brand and Android version. What you want is a back-up from your chosen data SIM to the other SIM.

What the control changes, and what it leaves alone

With the switch on, the phone may send mobile data through the line you did not choose, when the chosen line has no signal or is too weak. It does not move your voice calls. Calls and texts stay on the line you set for them. It does not turn Data Roaming on or off. Each line has its own roaming switch, and those stay as you set them. With the switch off, a weak travel eSIM stays the only line allowed to carry mobile data. If it has no signal, data pauses. It does not hop to the home SIM. That pause is the point. A minute without maps costs less than a home-network day pass you did not mean to start. The control also does not stop the home SIM joining a foreign tower. Joining a network is separate from using data. Calls and texts can still reach the home number. Some networks charge a daily fee for that, even when this switch is off. If you do not need the home number on the trip, turn that line off. Do not rely on this one switch.

The four settings that belong together

1. Choose the travel eSIM for mobile data
   On iPhone this is Mobile Data or Cellular Data. On Android it is the mobile data SIM. The tick should be on the travel plan, not the home network.
2. Turn data roaming on for the travel eSIM
   A travel eSIM needs data roaming on, because the network it joins is not its home network. This is the opposite of the home SIM.
3. Turn data roaming off for the home SIM
   That stops the home account being offered a data session. It does not block every call, text or voicemail fee on its own. Your network sets those rules.
4. Turn the switching control off
   That is Allow Cellular Data Switching on iPhone, or Auto data switch or Switch mobile data automatically on Android. Off means the travel line cannot fall back on the other SIM.

Read next: the roaming settings guide, how to set up an eSIM before you travel, what to try when an eSIM will not connect.