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".
When was that? My parent’s landline number stayed unchanged for 20 years. Through moves, too (with the caveat they stayed in the service area of the CO.)
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.