I had the same problem. US based PBX. UK located phone. The user has the phone in her home, connected to her DSL service. The only thing I could come up with to explain it was unused ports being closed after a given amount of time. The phone isn't used much. If she power cycled the phone, everything was fine for a while. After it sits for a couple of days, it fails. So now, if it has sat for a while, she's in the habit of power cycling it before she uses it.