Well, if the phone is not necessary anymore since you have another one then it is a wise move. The value of that phone will depreciate fast over time while the value of your satoshis will be worth more than double next year or by 2025. There is a risk of course but I guess most of us here are very confident of a major bull run during and after bitcoin halvings as what we've seen in the past.
-snip-
In fact, I think that it is all about
Opportunity Cost:
"the loss of potential gain from other alternatives when one alternative is chosen."
If you decide to sell the goods you don't need anymore (in a consumer society we usually have many), and you invest the price in Bitcoin, you have to assess first what assets you prefer. You should make the same decision when you think about other alternate investments also.
Instead or selling goods, idea I personally like as a first step towards a minimalistic life, you can also think about investing your time. But, again, your time has a value, so you have to decide which is the best opportunity among the existing ones (spend time, spend money, liquidate your goods, invest in traditional assets or altcoins instead, or simply save).