I doubt you would use ‘speaking’ or ‘talking’ at all in that case in German. I think you would just say ‘I'm in my car’ (‘ich bin im Auto’) or something similar. If you were actually speaking out of your car, instead, you could say ‘ich spreche aus meinem Auto’, but I doubt you'd use it in the context of speaking on the phone. Then again, I'm not a native speaker, so I might be wrong.
I think it's actually good. If you travel to a German speaking country, people will be using words in contexts and ways you are not used to from lessons. The ability to adapt and learn as you encounter unfamiliar phrases is crucial to language learning.
I also would not recommend relying solely on duolingo for lessons.
You can't always translate word by word. Particularly prepositions are used very differently in different languages. You should rather learn complete phrases. "to talk about" is "sprechen von" in German.
And btw., there are lots of different possible translations for "von", It can e.g. also mean "of".
Wir sprechen von meinem Auto is correct. I'm not sure if you can say We talk of my car, that would be the literal translation of the German sentence. Reversely, you can translate We talk about my car as Wir sprechen über mein Auto without any problems.
For me über and von etwas sprechen are equivalent.