It is also a communicator device, but only in the sense that for some silly reason I can't place calls with the 2000$ box on my desk, but only with the 800$ one in my pocket.
None of the apps I have are perfect, especially now that we are in an awkward phase where we have an excellent Material spec and are very far from it in practice. However, they all do their job well enough. So well that I don't remember the last time I added an app to my routines.
FXO card? :/
Of course you can. Get a Softphone[1], a SIP account[2], and you're good to go.
[1] e.g. https://itunes.apple.com/en/app/telephone/id406825478?mt=12
[2] Here's a good list (calls to USA landline & mobile are free on most of them): http://www.mobilevoip.com/supported_brands
Not to derail too much, 80-90% of my computer usage isn't even really possible with an iPhone or android, at least not productively. Small devices make it very hard for me to do anything other than small bite-sized interactions. When I'm writing music, prose, editing video, writing code, all of these things are borderline impossible on a phone, and barely possible on a tablet, and those things are the bulk of my computer usage.