Seems the first attempt to post here failed.
We have an L420s SwyxPhone on our tech bench that just recently started flashing lines 2 and 3 (2nd and 3rd buttons from top right corner speed dials). It started with line 2, but now it does the same thing for line 3.
The manuals I have don't mention anything about that, nor did the countless Google searches. The line buttons seem to work fine, but the lights keep flashing as if to tell me something is wrong. Problem is, I have no idea what else to check. Swyx logged nothing about the phones on its logs.