Millions of phones, there is bound to be a freakish incident or two.
In the late 1990s I forwarded information to the UK CAA regarding professional TV camera batteries (14volt 5 Amp hour Nicads) that were being re-celled by using solder rather than spot welds. the solder welds would fail and the strip of metal between the cels could slip and cause a short.
There were a couple of instances that occured on the ground where the nicad batteries shorted and caused fires. A 14 volt nicad battery can deliver an enormous amount of amps under the right circumstances.
CAA took no action. The TV industry self regulated and all pro camera batteries now have thermal fuses and there has been a trend toward lithium.
I cant recall a record of a fire in an aircraft caused by professional camera batteries although Im not sure if nicads in particular were ruled out as a potential cause in some of the unexplained accidents between 1980 and 2000 when their use was at a peak.
Mickjoebill