How to set default SIM for calls on Lenovo A7000 running Android Lollipop 5.0 and Lenovo's Vibe UI?
PS: I have bought an Android phone for the first time.
Open Stock dialer → Select the options from top right (Action overflow button - 3 dots after the voice search assistant).
Select Settings → Call Settings → Phone Account Settings → Make calls with.
Here you can select 3 options. If you select Ask first - you'll be prompted every-time you make a call, to select from which sim you want to make the call.
Or select either of the SIMs directly (using 2nd and 3rd options).