Purchases of mobile phone

I would also book it under telephone costs, I think that cost category is called "Mobile phone & internet costs"?

It doesn't really matter that much, as long as the professional use is estimated in a good way. You might want to consider that, perhaps the percentage could be a little higher? Standard is set to 20%, I set it a little higher myself after doing some research. And certainly for a mobile phone with call forwarding, I can imagine that it was purchased for professional use?