First the good news:
I can now reliably receive VoIP calls when I am registered to VoXaLot and I am using the Nokia VoIP client over a 3G connection. I was advised of the correct settings by another VoXaLot user, tmon, in the Whirlpool forum. Here is a link to that post:
Voip Data Usage - Mobile Data - Whirlpool Broadband Forums
And here is the verbatim advice from tmon:
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"My Nokia VOIP is registered with Voxalot... settings below
Service Profile: IETF
Default Access Point: Mobile Internet
Public Username:
VoxalotUserID@au.voxalot.com
Use Compression: No
Registration: Always On
Use Secirity: No
Proxy Server:
Address: None
Realm: None
Username: None
Password: None
Allow loose routing: Yes
Transport Type: Auto
Port: 5060
Registrar Server:
Address: au.voxalot.com
Realm: voxalot.com
Username: VoxalotUserID
Password: VoxalotPass
Allow loose routing: Yes
Transport Type: UDP
Port: 5060
After the above if everything went okay, you should be registered - Remember that the Back button saves your changes. But to get incoming you need to do the next step outlined below
Goto Tools --> Settings --> Connection --> Internet Tel.
Create a new profile (if there isn't one already) and call it whatever you want (or leave default name "Default")
Under SIP profiles select the voxalot account you created above.
And that's it. Remember to use the Back buttons as that's how the settings get saved. Otherwise they are discarded."
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Now for the not so good news:
If you have a regular speed 3G connection, ie NOT a high speed (HSPDA) 3G connection, you should do an upload speed test to see what your upload bandwidth is.
The reason for that test is simply due to the fact that the Nokia VoIP client by default utilisies one of the G.711 codecs, but I am not sure which one. Anyway, a G.711 codec needs 64kbps to function properly. So, if your upload bandwidth is less than that you should either change over to fring which uses less bandwidth, or the settings in your Nokia VoIP client should be changed to only allow the use of G.729.
Two days ago I changed over to the Three 3G network. I have only been able to get an upload speed of 56kbps on Three. When I make outgoing VoIP calls, the outgoing voice from my phone is terrible, but the incoming voice is fine. When my wife calls me using an ATA, which is set to only use G.729, and is logged into another VoXaLot account, both sides of the conversation are fine.
It seems to me, therefore, that the codec settings of any Nokia VoIP client need to be changed to G.729 only, where there is less than 64kbps upload available to the user of that client. (Some 3G phones, such as my Nokia E70, are not capable of the higher speeds available from HSDPA, and some networks may throttle upload speeds.)
Hopefully Martin will be able to see his way clear soon to help me with the codec change to my Nokia VoIP client, as changing codec settings is not able to be done via the client's user interface.