For most of us, making and receiving phone calls with our devices is something we simply take for granted. Unfortunately, Nexus 6 owners who use Sprint as their carrier are finding that it's not quite that simple.
Background
The Nexus 6 works on all five major US carriers with the simple swap of a SIM card, but that doesn't mean that all problems with the device are universal to all carriers. In this case, a lot of people who have a Nexus 6 on Sprint are reporting that their devices are not receiving phone calls. The problem seems very random and there isn't much of a pattern to it. I have experienced this personally on my own Nexus 6, and the only pattern I can find is that, at least for me, it only happens when my phone is connected to Wi-Fi. I'll go hours without receiving phone calls, then it'll work fine for the next day.
To make matters worse, when the phone isn't getting calls, its behavior mimics that of the phone being turned off. Callers are sent right to voicemail and you don't get a missed call notification. The only way you'll know that you missed a call is if the caller leaves you a voicemail.
The Cause
So far, neither Sprint nor Google seem to have any answers as to what's going on. Sprint's tri-band Spark-capable phones (of which the Nexus 6 belongs) have a long history of having issues with receiving calls, but that's always been due to something on the network side, referred to as circuit-switched fallback, or eCSFB. That does not appear to be the issue in this case, as I have other Spark devices that do not have this problem when connected to the same towers as my Nexus 6. This has all the signs of being a radio issue with the device, especially because it only seems to happen when connected to Wi-Fi.
Workaround
In my testing, the only workaround I can find is to leave my Nexus 6 on LTE full time. As long as Wi-Fi isn't connected, I seem to receive phone calls without issue. If you absolutely need Wi-Fi, though, the only workaround I can recommend is switching carriers.
Closing
I can't imagine Google and Sprint will let this bug linger for very long, as most people would consider the ability to receive a phone call to be critical. Hopefully we'll see a fix for this in the next device OTA, but that remains to be seen.