Does it happen on only one line, some lines, which lines?
Only on one module, some modules, which modules?
Keep ringing after the call is answered indicates it rings forever, correct assumption?
If some phones, swap phones that do not do this, and report.
If some lines, swap lines that do not do this and report.
If some modules, switch modules and report.
You do not always get what you pay for, but you never get what you do not pay for.