Had this problem 1 week after buying it. Swapped the phone with another HTC One M8 and worked for about 20 days. Same issues again. Cannot call out or receive calls, if i do the call will be dropped shortly after. Beyond frustrated with this issue. I've read about the LTE and spark issues on sprint w this phone. Best Buy and sprint keep saying this isn't a known issue. Switching to CDMA worked great for 1 whole day. My 14 day return policy is up at Best Buy so I'm stuck with this phone....What do i do now?