Where is area code 173?

There is no area code 173 in North America (USA, Canada, etc.). North American area codes cannot begin with '1'.

+173 as an international prefix is telephone country code +1 (North America), but is an incomplete area code. You need the three digits after country code +1 to narrow down the location any further.

There are other countries that may have a domestic area code 173 or similar, but it's impossible to tell which one you might be asking about.