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In website, Sony Mobile says:
"Where both worlds can always reach you
Business or private call? Separate work and play with dual SIM cards. You can keep both SIM cards active at all times – meaning you can take calls on one SIM, and still receive calls and messages on the other."
So, Z3 Dual: Dual SIM Dual Standby or Dual SIM Dual Active ?
When you have an active call on one SIM card, the other will be blocked. However, there's an imbedded option called "Dual SIM reachability", which forwards calls from the inactive SIM to the active one.
The device is dual standby,but the option Quester mentioned should help.