Hi maro21, "contact:phone" or "phone" it's only a difference of form and not of substance: the data it's always the same, this is the most important thing.
Here:
https://wiki.openstreetmap.org/wiki/Key:phone
says that "phone" and "contact:phone" are the same thing but the second one is less used.
Here (key "facebook"):
https://wiki.openstreetmap.org/wiki/Key:facebook
the criteria it's the opposite of the "phone" 's one.
Watching on taginfo, this is the "de facto" situation:
https://taginfo.openstreetmap.org/search?q=phone
"phone" is more used than "contact:phone"
but instead "contact:mobile" is more used than "mobile"
https://taginfo.openstreetmap.org/search?q=mobile
and "website" is more used than "contact:website"
https://taginfo.openstreetmap.org/search?q=website
but again "contact:instagram" is more used than "instagram"
https://taginfo.openstreetmap.org/search?q=instagram
and so on....
It's look like the most "recent" tags (facebook, instagram, ecc.) have more use with the "contact:" prefix but the "old" one ("phone", "website", "email") have more use without the "contact:" prefix.
So I discussed with other mappers about this argument and finally we think that try to uniformate to an only one standard with the "contact:" prefix it's not a bad idea.
That's all.
What do you think?

and so on....
