With the "It's 2018" clause, both mean more or less the same thing (as Neil says). But without that clause to clarify, the implication would be quite different.
I still use this phone would be something you say to emphasise the fact that generally-speaking, you still use the phone. If someone suggested you throw the phone away, for instance, you could say "I still use it", or if you wanted (as in your original example) to emphasise that despite it being old (or perhaps broken in some way) you still use it.
I'm still using this phone, on the other hand, more emphasises the fact that you are currently in the act of using the phone — not just that you use it from time to time. For instance, if someone has lent you their phone, and they ask for it back, you could say "I'm still using it" to indicate that you haven't yet finished with it, but you'll probably finish shortly.