Add `phone` in international format

[habi]

The phone key of a POIdefibrillator should added in international format.

While correcting some wrong values [^1] in my vicinity I'm seeing a lot of defibrillator which don't use the international format. Would it be possible to add this to the phone number automatically?

[^1]: and stumbling over a bunch of defibrillators in the wrong place: https://github.com/chnuessli/defikarte.ch-app/issues/158

[chnuessli]

This was deployed, long time ago :) sorry, never closed the issue.