Mobile phone purchases with Proximus

If you're talking about €499, I would just put that directly under expenses. Not as an investment, just as 'mobile phone & internet costs'.

I think the amount is good enough :)

I wouldn't record it as an investment either. On the one hand because the amount is low, although I think that's only part of the answer. But also because you can give an explanation (I think) that your mobile phone won't last three full years. Most smartphones don't last three full years anyway.

Then I wouldn't write it off, but just treat it as expenses.