Dell doesn't design hardware, it assembles it. Mobile phones are more about hardware design than assembly. Thus, it doesn't play to what Dell does at all.
Dell doesn't assemble hardware. They outsource to some Chinese producer like everyone else does these days. This is why they've sort of lost their relevance.
They've always done that, the reason they're not doing so well is mostly because the margins for commodity hardware has gotten really thin. There was a time when CompUSA & Dell were thrived on computer sales.
I don't think that's true. There was a time in the not too distant past that many of their computers were assembled just in time in a suburb of Austin (it was still operational circa 2001).
Edit: Dell continues to build servers in Austin, but most everything else is outsourced.