If replacing the set fixed the issue, the problem was likely a phone problem, odd but still undeniable.
If you moved the phone from a 3300 to a 5000, it should have reflashed the firmware, which in itself could have "fixed" the problem, now moving it back to the 3300 will flash fiirmware again...
In other words, if the firmware was the issue, your troubleshooting procedure could very well have been the actual fix.