Hi this is a phone line at a commercial premises. We use the line to take orders constantly throughout the day and this really affects business.
I randomly have got this error 3 times now. There is no dial tone so you cant dial out but if you call in, it just rings and rings. I have changed the phone twice, and have called engineers out who could not find a fault. It just so happens that the line somehow corrects itself after a day or 2 and the engineer doesn't really have anything to look at by the time they arrive. The last time a senior engineer came, couldn't see what the problem was, and said it must have something to do with our alarm system interfering with our line, and suggested we call them out. We called them and they couldn't see what the problem was. They both have suggested if this happens again, we plug in the line directly into the test socket. This hasn't fixed it.
The phone itself was tried and tested and works fine in the shop next door.
I'd like to take some time to welcome you to the forums and say that I'm really sorry to hear that you've been experiencing an intermittent issue with a loss of dial tone. Intermittent faults can be really difficult to diagnose, as chances are by the time a technician can be at the premises, then the fault has cleared, as you've found out.
I do believe that you've done as much as you can with the diagnosis on this, trying different phones, trying your phones in another location etc.
Fixed alarms can cause issues on times, but you've also had this checked out. I can only suggest having to book another technician to call out, hopefully we will be able to detect the issue.
I see that you mention that this is a commercial premises, so I'd have to ask you to report this to our Business Department, you can contact them on 0800 052 0800 so we can escalate this again. Should the account be a residential account, could you please let me know.