How to forward calls in Android without extra application?
I got the answer in Oneplus support chat


  In that case, please Go-to phone app-> click the 3 grey dot right top corner-> click settings -> call setting-> select SIM 1 or SIM 2.


Output: as expected. 
Details


Set your Smartphone to Airplane mode or switch it off if you want to forwarding to go directly to your second phone. Setting: unreachable call.