Does NFC work while the phone is doing something else?
Usually the read itself works. What changes is whether you notice and whether anything happens, because a tag read during a call or a full screen video competes with whatever already has your attention and the system decides which wins.
The read usually works
Detecting a tag is a low level operation that does not need your attention. A phone in the middle of a call or playing a video is still looking for tags under the same conditions as usual.
So the exchange happens and the content arrives at the system.
What is uncertain is the next step, which competes with whatever is already in front of you.
The action may not
A full screen application, a call, or a video may not be interrupted for a tag, or the result may arrive as a notification you do not see until later.
On iPhone the background path presents a notification anyway, and a notification arriving during a full screen activity is easy to miss entirely.
So the honest description is that the tag was read and the outcome was deferred or dropped, rather than that the tag failed.
Behaviour varies by platform, by manufacturer and by what exactly is running, which makes it a poor thing to depend on.
When an app is claiming tags
On Android, an application in the foreground can claim discovered tags. If that application is not the one you expect to react, the tag goes to it and nothing you were expecting happens.
This is the confusing case: the tag was read, an application received it, and that application did nothing visible because the tag meant nothing to it.
It also explains a tag that behaves differently depending on what was open, which looks like an unreliable tag and is not.
What to do
Get to a neutral state before tapping: home screen, phone awake, nothing full screen.
Do not build a flow that expects a tap to interrupt something. Anything that has to work reliably should be tapped deliberately by a person who is paying attention.
When testing, close everything first. A tag tested with a reading tool open is a tag tested under a different rule from the one visitors will meet.
And treat a tap that produced nothing during a call as untested rather than as a failure, because the read almost certainly worked.
Frequently asked questions
Can I tap a tag during a call?
The read generally works and whether anything happens depends on what has your attention. Do not rely on it for anything that matters.
Why did nothing happen while a game was open?
A full screen application may not be interrupted, or it may have claimed the tag itself. Both leave you with a tag that appears inert.
Does the phone queue the tag for later?
Sometimes the result arrives as a notification you see afterwards. It is not a queue you can depend on.
Should I close apps before testing?
Yes. A foreground application can claim tags, which changes the behaviour you are trying to measure.