(Note: I haven't actually tried this recently).
The master branch is always a work in progress, so there's potential for instabilities. That's especially true in the domain of telephony, which receives a fairly steady stream of AOSP contributions. Telephony in the master on actually hardware tends to be hit-and-miss, because the proprietary telephony binaries tend to be tied to specific versions of the Open Source code, and getting them mismatched commonly results in having no telephony functionality.
JBQ