A few weeks ago, the Google Duo team rolled out the option to place a video call through Assistant. All you had to say was "Ok Google, video call contact_name" and Duo would open up and make the call to that person. However, at the time, I thought the feature wasn't live for me. Assistant would tell me it's calling the correct person, Duo would open, but it would stay on the main screen, not placing the call. Clearly, something was amiss and I thought it was a bug that would be fixed with time. It wasn't until a few days ago that I figured out the reason it wasn't working: my contacts had their numbers either saved without a country code or with 00 as the international prefix.

If you haven't lived abroad, you may not be aware that international call prefixes vary by region. I only learned that while researching this piece. I've lived in Lebanon almost my entire life, and in France for a year. Both countries use 00 as the prefix, so I would dial "00 country_code contact_number" if I was making an international call. Turns out some countries don't use the 00 prefix. In the US and Canada, for example, you dial 011 followed by the country code and number. Japan, Australia, Russia, also have their own different codes. It's all explained in this Wikipedia article.

The universal symbol that all smartphones understand is the plus + sign. If your contact is saved as "+ country_code contact_number," the phone will figure out what the "+" is supposed to be based on your current location. So the number you've saved will work whether you're in France where they use 00 or in the US where they use 011.

And this brings us to Google Duo. Regardless of the way you've saved your contact's number — without a country code (just a local number, no prefixes), with the country code and your country's international prefix (00 or 011 or 010, etc), or with the auto-generated + symbol — Duo will find that person and video call them. This works inside the app.

However, if you want to video call someone on Duo via the Google Assistant, whether it's on your phone or your Smart Display, it seems that Assistant gets a little picky about who it can dial. For me, it wouldn't call most of my contacts (saved as local number or with 00 as prefix) even though they were all reachable inside the Duo app. It's worth noting that this may depend on the country you're in and the language you've set Assistant to — I'm in Lebanon but set it to US English, as all of us do when we want the latest features. I can't test it, but it may be that Assistant understands 011 as international prefix when it's set to US English, or 00 when it's set to French for example.

Given the intricacies of this and the fact that Google doesn't explain them anywhere, the best bet is to just save the numbers with the + symbol and the country code to avoid all headaches. I only made that small change to the contacts I often call on Duo, re-synced them (via Android's Settings -> Accounts -> Duo), and now I can ask Assistant to video call them and it does so. This works both on my phone and my JBL Link View Smart Display, which I'm reviewing soon. Whew.

So to recap in a succinct way:

• Number saved as: "+ country_code contact_number"
  • video call on Duo: works
  • video call on Duo via Google Assistant: always works
• Contact number saved as: "local_contact_number" OR "any_international_prefix country_code contact_number"
  • video call on Duo: works
  • video call on Duo via Google Assistant: may or may not work, depending on your location and Assistant's language setting

It's a simple solution to a very annoying problem that Google's support documents don't mention anywhere. So if you've been trying to get Assistant on your phone or Smart Display to video call someone and it fails to do so, even though you can easily call that person inside the Duo app, double check the way you've saved the number.