Some of you exclusively use their Google Voice (GV) numbers, some have differentiated between friends and colleagues by giving their regular number and GV number separately to each set of contacts, and some might use the GV number occasionally for a few contacts only. Regardless of your case, sending texts from that GV number has been pretty hit-and-miss when it comes to Android Auto, Android Wear, and Assistant. But that should be fixed soon.
As explained by several complaining users in this support thread, if you have a Google Voice number and you set it as the default SMS number in Hangouts then set Hangouts as your default SMS app, when you're using Android Auto and you say, "OK Google, send a text," the command will forego your defaults and just issue a regular SMS text through your carrier number. (Some also report that the text doesn't even show in Hangouts, but in the phone's pre-installed SMS app.) That can get quite problematic if the recipient doesn't have your carrier number and wonders who's texting them.
Several other users chimed in saying it also happens with Wear and regular 'ol Google Search and Assistant, and even verified that it only happens with new texts whereas replies are sent correctly from their GV number. However, one user went the extra mile and pinged Jan Jedrzejowicz, the product manager for Google Phone and Google Voice. In reply, Jan said that, "OK Google, send a Google Voice message," will be coming soon to Google Voice.
'OK Google, send a Google Voice message' coming soon to GV, this will help
— Jan Jedrzejowicz (@JanJedrzejowicz) September 5, 2017
While this may not be a complete solution to the Google Voice number vs carrier number dilemma on Android (the GV voicemail for example isn't recognized by Auto when you ask for your voicemail), it does at least provide a way to issue new texts using your GV number without having to dig out your phone and type them manually. It should also technically work on Wear and Assistant as well. You'll have to remember to specify "a Google Voice message" instead of "a message" each time, but it's what I do daily when I send WhatsApp messages. The same is true of Telegram, Viber, WeChat, and a few other messaging platforms. You get used to it.
It's a bit sad that Google can't find a way to let you completely set your Google Voice number as default for everything, but well, integration of its different products hasn't always been the company's strong suit.