Voice Control Offline Is there a way to do "Ok Google" offline?
Basically I do not have a data contract.
For that you will need the Google Now offline voice recognition.

Go to settings >> Offline speech recognition (download latest supported language files).

You can perform some offline voice actions (play music, turn on wifi, turn on flashlight) 

Here is a screenshot of Google assistant offline commands:



To make a call while offline:


Tap the mic on Google assistant
Then say Call "xx" where xx is any contact
After this it recognises the voice and identifies the contact and immediately calls the contact


See below screenshot:



(click images for larger variants)