What phones do you have? There was at one point LTE devices had an extra radio and antenna to do both, but it drained batteries quicker and such. Then they opted for AC and devices didn't have the extra hardware. Depending on the iPhone model you may have a legacy device where you can't do both.
Any iPhone on Verizon from the iPhone 6/6+ Series and newer will be able to use Data and Voice at the same time assuming the following are both true: