Samsung Galaxy S6 on Verizon - capable of simultaneous voice and LTE w/o having to use VoLTE? Does the Samsung Galaxy S6 on Verizon support using 4G LTE data while on a voice call without having to use VoLTE mode? It seems like a lot of newer phones don't support this.  VoLTE might be fine if in an area with uninterrupted 4G signal, but last I read, switching to 3G while on a VoLTE call will cause the call to drop.
No, simultaneous voice and LTE data is only available when using VoLTE (aka "Advanced Calling") on the Samsung Galaxy S6. See the Tech Specs on Verizon's store page for the S6 under the ADVANCED CALLING 1.0 heading:


  Activate Advanced Calling 1.0 to experience Simultaneous Voice &amp; Data


Verizon also accounts for this with a note in their Advanced Calling 1.0 for Android Devices FAQs under the question What is Advanced Calling?:


  **Note: Most 4G LTE Android smartphones today already support simultaneous voice and LTE data without Advanced Calling 1.0. However, some newer Advanced Calling 1.0-capable devices such as the Droid Turbo by Motorola do not support simultaneous voice and LTE data unless Advanced Calling 1.0 is activated.


The Galaxy S6, it seems, is one of those newer devices that doesn't support simultaneous voice and LTE data without Verizon's Advanced Calling service. There seems to be speculation that this is because of a lack of a second LTE radio but my cursory Googling didn't pull up any primary sources.