The pass starts more easily than it stops
A daily travel pass is a roaming product. Some of them start for the day as soon as the phone uses a small amount of data, then charge that day even if you opened one map. Background app updates, a photo library backing up to the cloud, and a video that auto-plays all count as use. The phone can do each of those with the screen locked.
When the allowance inside the pass runs out, later use can be billed at an out-of-bundle rate. Carriers change those rates. Check the current page from yours before you travel, and do not rely on a number you remember from a previous trip. This site does not print carrier prices.
Calls you never answered
Answering a call abroad can be a roaming charge. So can a call that goes to voicemail, if your carrier treats the forward as a call while you are roaming. Call forwarding to another number has the same shape. If you do not need voice on the home line during the trip, ask the carrier what "roaming off" covers for calls as well as data, and turn off forwarding you do not use.
Plane and ship networks are a separate trap. A cabin network can present itself as cellular service and sit outside a normal travel pass. Airplane mode, with wifi turned back on if you want the plane's wifi, keeps the phone off that cellular path.
What to set before you fly
- Data roaming off on the home SIM. On an iPhone, check each line. On Android, open the SIM that holds your home number and confirm the roaming control is off.
- Low Data Mode or Data Saver on, and photo backup paused in the cloud app. Those settings reduce accidents. They are not a cap.
- A spending cap or a roaming bar, requested from the carrier while you can still reach them cheaply. The control has a different name at each company.
- A travel plan for data: an eSIM installed at home, or a plan to buy a local SIM on arrival, so you are not tempted to flip roaming on for one map.
The reference page for the full list is Roaming and bill shock.