First point is to check coverage and signal strength. You may have a change with your “anchor” (G)MSC. As you have a secured TCH end users may be suffering from fading, etc..
Check if there was any recent Cell Planning and whether any changes were made to the RF Network.
As you have ZTE, Ericsson and Nortel (CDMA??) check for the last set of integration / Handover test results and whether any issues were flagged.