I have had a google to try & find a solution, but none has presented itself.

When I’m in my car, I have my phone connected via bluetooth, & to make a call I use OK Google – I tried this the other day & it wouldn’t let me make a call unless I download an app called Duo.
I can make & receive calls over bluetooth if I use the touch screen.
I cannot see a way to circumvent this…

Any ideas?

Yep..download the app…

The only time I need the app is when I use voice command – why won’t it work without it?!

I don’t see any need or point to the app

I had this issue a few versions of Android ago when using Android Auto. An update fixed it, was probably over a year ago.

What version of Android are you using?
Have a look in the voice settings and Hands-free and see what the allow requests say. Maybe it cannot have requests when the device is locked?
Also maybe an idea to play around with Smart lock to make sure the phone isn’t locked when connected to your car and see if this helps?

Highly likely to be what services you have setup for Google assistant to use.

Guessing you are on a recent version of Android with Google assistant. Go to Google assistant settings, services, voice and video settings and confirm that Google assistant can use your telephone.

I’m guessing it is trying to use duo because there’s no number linked.