WiFi or mobile data?

Voice is one of the lightest things a phone does. At roughly 30 to 50 kilobits per second in each direction including overhead, a minute of calling is about half a megabyte. A ten-minute call is around five megabytes. An hour is roughly thirty.

For comparison, one minute of standard-definition video streaming is in the region of ten megabytes, and a single modern web page with images can be two or three. A long phone call is cheaper in data than reading a news site for a few minutes, which is not most people's intuition and is worth knowing before you avoid a call to save data.

This is also why a slow connection is not disqualifying. A 3G connection carries voice comfortably. What breaks calls is not scarcity of bandwidth but variability in delivery — packets arriving in clumps rather than at an even pace — and that is a property of the network's congestion rather than of its advertised speed.

This is the point of using an app abroad, so it is worth stating precisely. When you dial through the app, your phone sends data to our servers and we hand the call to the telephone network in the destination country. Your own operator sees data traffic, not a phone call, so international call rates and roaming voice charges do not apply.

What does apply is your data tariff. On a plan that includes roaming data, or on a local SIM or eSIM, the cost of a call is the half-megabyte-per-minute figure above at whatever your data costs — usually a rounding error. On a plan with no roaming allowance, data can be billed per megabyte at a rate that is worth checking before the first call.

The saving is generally large in either direction, because operator international and roaming voice rates are typically an order of magnitude above app calling rates. Our published rates start at $0.03 a minute, and the comparison against what a roaming call costs is not close.

Make the same call twice to the same number, once on WiFi and once on mobile data with WiFi switched off. It takes two minutes and it is worth more than any measurement, because it changes exactly one variable while everything else — the destination, the route, the time of day, the person on the other end — stays fixed.

If mobile data is clearly better, the WiFi network is the problem. On a network you control that is worth fixing: move closer to the router, use the 5 GHz band if there is one, or stop whatever is uploading. On a network you do not control — a café, a hotel, an office — there is nothing to fix and mobile data is simply the answer.

If WiFi is clearly better, use it and turn mobile data off before long calls to avoid the phone switching mid-call. If the two are indistinguishable, use whichever costs you less, which abroad is usually WiFi and at home is usually neither.

And if both are equally poor, the problem is not on your side at all. That points at the route or the destination network, and it is worth reporting rather than troubleshooting: send the date, the time, your time zone and the number to [email protected], because that is a case we can actually look at.