Hello. We have several SPA504G phones. Same network without NAT, same PBX. Some units accept simultaneous calls to all four extensions and reply with four "Ringing" messages. Other SPA units accept only one call to any ext and reject more that one call with "486 Busy Now". All settings on SPAs are ~identical, firmware versions are identical, factory reset and firmware re-uploading don't solve this problem. Here's XML config from "bad" device.