If you are using a S7 and S7 Edge, S8 and S8+, then there is some good news. It seems you do not need to assign a particular line to call a number in the Contact itself. If you just dial a number and a line is not assigned with it to dial, the pop up will come and once you choose the line in pop-up, Samsung device is storing that choice in the contact and associating the line on the fly. I believe the latest update which has upgraded the device software to July 1, 2017 patch made this happen. Please keep posted how you guys see at your end.