It would appear that LMY48I (which has a fix for Stagefright) is wreaking havoc on Nexus 6 phones with T-Mobile USA SIM cards. The phone simply refuses to connect to the cellular network, and thus calls and text messages are out the window. This issue seems to mainly be affecting people who upgraded from LYZ28E (T-Mobile USA's unique build with Wi-Fi calling) to LMY48I by flashing factory images. This likely has to do with the fact that the Wi-Fi calling-enabled radio firmware used in the LYZ28E build is quite a bit different than the "standard" radio firmware used in the more mainstream LMY47Z build. When a device gets flashed with the specialized radio firmware, it's quite possible that going back may be causing something in the system to freak out.
Without a doubt, one of the great things about the Nexus line is that you have the option of flashing factory images when new builds come out rather than waiting for an OTA. With that said, just because you can doesn't mean you should. This is especially true of the Nexus 6, which has sported carrier-specific builds for almost its entire life thus far. With that in mind, T-Mobile announced today that its Nexus 6 Stagefright fix will come in the form of build LYZ28J, which will also keep Wi-Fi calling intact (LMY48I doesn't have it). This is why it pays to be patient rather than just saying "ERMAHGERD! URPDERT!" every time you see a new build released.
If you're experiencing the cellular connectivity issue on LMY48I, my suggestion would be to flash back to LYZ28E (the build you're actually supposed to use) and wait for the LYZ28J OTA or factory images.