Is there any (open) standard for importing/exporting contact from/to any mobile phone?
That format most likely is VCard (Versitcard), which even has its own mimetype (text/vcard). According to the linked Wikipedia page, it became a standard around 1996 (and since it was proposed 1995 for the first time, older devices won't know it).

Android's contacts app uses this format when directly exporting your contacts from it. To my knowledge, this format is also used when transferring contacts via Bluetooth (even to/with older devices not necessarily running Android or being "smart phones"). Mail clients support it as well. So if you're looking for any standard here, this most likely is it.