I remember the happy days when a telephone number (also an email) was a ephemeral, changeable thing. Not a permanent unchanging form of ID.

Alas, if you move out of the country for a period of time, it is a challenge to maintain the host country phone service. So there are no guarantees that you can hold on to that signifier of "this is where I was the last time I wanted to get a new phone subscription".

So the tattoo might be the better choice.