how can i choose a sim to make a call if 2 sims are inserted in lumia 730?

I am using lumia 730 upgraded to win 1o.now i have inserted second sim but cannot choose from calls can be made. The 1st sim is automatically choosen to make calls. I tried to make calls using my second sim, which is newly inserted, but all went invain. Anybody please tell me what i have to do?

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