On a phone it's almost always just a dialtone for a modem. Though my last company switched to a voip system and the phones all had the ethernet connections for the desk in them, but I have not seen them in hotels (though I don't look either)

When in doubt, ask and hope the sixteen year old at the front desk actually has a clue.

On a lamp, desk, whatever it could be either ethernet or a phone line if it works at all, if it's a smaller jack it's a phone line, if it's the larger one it could be either.