This aspect of grammar is called the distributive plural. Swan in Practical English Usage (p530) has the following discussion:
Singular and plural: distributive plural
1. people doing the same thing
To talk about several people doing the same thing, English usually
prefers a plural noun for a repeated idea.
Plural forms are almost always used in this case if there are
- Tell the children to blow their noses. (not ... to blow their nose.)
- Six people lost their lives in the accident.
Quirk et al. in A Comprehensive Grammar of the English Language (p768) list two similar examples:
and agree with Swan that "... the distributive plural is the norm ...".
But the CGEL goes on to state that:
... the distributive singular may also be used to focus on individual
instances. We therefore often have a number choice.
The CGEL concludes its discussion as follows:
The singular is sometimes used to avoid ambiguity:
- Students were asked to name their favorite sport.
The singular makes it clear that only one sport was to be named. Similarly:
- Children must be accompanied by a parent.
Turning to the OP's example, the speaker has a 'number choice'. While, according to Swan and Quirk, the plural is the more usual form (People are using cell phones), the singular (People are using a cell phone) can also be used 'to focus on individual instances'.
As for the recipient of the message, their world-knowledge will most likely lead them to interpret both sentences identically, namely that each person is using his or her own single phone. People don't usually use more than one phone at a time, and people even less usually jointly and simultaneously use a single phone.
The same reasoning applies to the backpack example. Our experience of the world tells us that people almost always carry a single backpack and almost never share the carrying of a single backpack.
It is incumbent, therefore, on the maker of the message to anticipate when our real-world experience may lead us to the wrong interpretation or when the message is inherently ambiguous and a correct interpretation is important. In both such cases, the message needs to be phrased in such a way as to be clear to the recipient how many of the items are involved for each of the people.
Two people and one backpack: They are carrying a backpack between
People, all using more than one phone: People are using each of their