How to forward calls in Android without extra application? In Nokia, I did it by typing *71003584012345678 where 71 for the unanswered calls, 00358 for the Finnish country code and the phonenumber 4012345678. 
I know there are many applications for the task but I do not want to use them. 

I heard that some Nokia Android phones and Sony phones have a builtin feature for the call forwarding in their phones. 
However, I cannot find such a feature in my Oneplus.  

Phone: Oneplus 3T
OS: Android 7.1.1
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.