A number of forces are acting on a system. If net external force on the system is zero, Then 

If net external force is zero, then linear momentum of the system must be conserved.
If external  forces are not concurrent then , net external torque will not be zero and in that case angular momentum of the system will not be conserved 
If external  forces are concurrent then net external torque must be zero and in this case 
angular momentum remain conserved.