Inconsistent Incoming Calls

[cstewart]

I have been happily using the Voxalot service for many months basically just for outgoing calls (using VoipCheap). Now that I will be travelling more I want the incoming calls to work so that when away in hotels I can call home.
My home setup is a computer and an ATA (DLINK DVG-2001S) behind a DLINK DI-524 router. I have STUN enabled on the ATA (Full Cone NAT).

I have tried using Gizmo Project softphone on my computer (making sure the SIP and RTP ports on the softphone are different from the ATA) to call my voxalot number on the ATA. The result is I either get forwarded to the Voxalot voicemail or the softphone stops ringing after approx 25 seconds. I get no ringing on the ATA.

I also tried with only the ATA behind my router to call from a Sipbroker PSTN number (Australian gateway). The result is either it works perfectly (about 1 in ten times) or I get an engaged signal (most of the time). One time I got the voxalot voicemail.

I have tried putting the ATA in the DMZ instead of using STUN and also tried a different ATA (an SMC travel gateway which I intend to use on the road) and also with two different voxalot numbers all with the same results.

I understand from the forums that the most likely problem is with the router but I would expect the results to at least be consistent.

Has anyone else experienced this inconsistency and if so what is the fix?

I just this minute tried to call from the PSTN gateway and it worked perfectly twice in a row, which is a first. Were there problems with voxalot about ten days ago when I first started testing or was I just lucky this morning?

Any comments would be appreciated as I am travelling next week and want some confidence that I can call home reliably.