a) that T-mobile LG Flex is not compatible on Sprint. You'll have to get a new phone (sprint has it's own version of LG Flex)

b) Sprint's network is not physically capable of doing talk+data at the same time. When you start a call on any Sprint phone, the phone drops LTE connection and uses CDMA/1X voice. CDMA/1X voice does not allow data to flow at the same time
(GSM/HSPA allows it. that's why you can do talk+data on T-mobile on 2G/3G without VoLTE)

Non-spark phones can do talk+data because they have a second antenna (one antenna to maintain CDMA/1x voice, one antenna to maintain LTE connection). New spark phones only have one antenna, so they can't do this trick

Verizon gets around this by deploying VoLTE. Phone stays on LTE, voice call goes over VoLTE on LTE network, data can still flow over LTE. Sprint doesn't have VoLTE at the moment.