No idea.

If you are really curious, you might want to try asking in the FWD support forums (registration required for the FWD forums, as they have a lot of jerks that like to SPAM them):
FWD User Forums :: Index

It is not necessary to put the * in front of the dialed US toll free number. Since SIPBroker's default (if you don't dial a prefix) is to route the call via ENUM, calls are terminated via one of the three partners on ENUM who have registered to handle toll free calls.

Don't believe me? I just tested it by dialing into SIPBroker's Anaheim, CA, access number and dialing 1-800-555-8355-#. In a few seconds I was greeted by TellMe's welcome message.

By not dialing * first, you will not bump into Sipura/Linksys' "Use RTP Encryption" feature code (*18).

I tried this also for TELLME but I am finding that DTMF is very unreliable. I can't get it to work at all via the default ENUM providers but via SIPBROKER DTMF does work - I wont say it is completely reliable though. If you remove the *18 from the regional settings on a sipura it wont be a problem. I chose however to let my dialplan put the * in, I just dial 1800

Why sometimes the call can go through, but sometimes it doesn't work, just busy signal.,Even I dial the same number. Anybody knows?

The network is overloaded, somewhere between you and the POTS gateway, also, if you "peer" one network to an other, one of them is overloaded, or, the SIP (protocol) message did not came 100% through, so best chance you make, when you minimal peer, aka weakest-link...:(

How would I add this toll-free check into a Voxalot dialplan? I've got a provider setup to take all US 10 and 11 digit calls, but they charge for 800 calls.

I was hoping to set up something that would catch the 4 toll-free area codes, but I can't think of a way of doing it in one check, only 4 different checks. The closest I got was:

_18[0678][0678]X.

But that hits non toll free area codes too (e.g. 1806...).

You will have to use 4 different checks. ENUM should be catching toll free numbers if it's enabled, but to force free toll free calls I have the following in my dial plan:
Just make sure these are ranked before your regular calls.

Toll free in other countries
 

I have allowed for toll free calls to Germany, Luxembourg, the Netherlands, Norway, USA and Canada using instructions found on the wiki, as well as those in country code 800. Being in the UK, I have no problem there.

I am wondering whether I might be able to reach toll free numbers in any further countries, notably Belgium, France and Ireland. I haven't seen any way to do so yet, but if it has been found, could someone please advise?

Toll-free for Netherlands, is not working anymore, so,*393*31800....
will give no connection, the "XS4ALL provided" message can still be heard, then no connection....:(

Don't know.

However, one thing that you might try, is to see if there are any VoIP providers who are located in (and specifically service) the countries that you are interested in. And, if there are, look over the cost info carefully on that company's web site, to see if they include toll free calling "for free" as part of their service (or if you have to pay for it). And if they do include free "toll free" calling (for a country you are interested in), consider signing up for a minimum prepay account with that provider (assuming the minimum isn't too high to make it not worthwhile).

i.e. If you can find a VoIP company that has (as part of their package) the toll free calling you want, you should (in most cases) be able to set them up as a provider in VoXaLot, and route your toll free calling (for that country) to that provider. Of course, this will mean you will have to create an account with that VoIP company, and many VoIP companies have a minimum prepay amount for opening an account. But if that prepay is "low enough" (and you don't get socked for things like monthly fees), it might see be "worthwhile" if that gives you "free" access to the toll free calling you are looking for...

BTW: I actually do something similar with my SIPphone/Gizmo5 account. While I don't use it for calling "toll free" numbers per se (as I have other ways to route the toll free numbers I want to call), I do use it to make "free calls". Specifically, the main use I have for my SIPphone account (which currently has zero balance) is simply to let me make free "Backdoor Dialing" ( Gizmo5 - Make free internet calls from your mobile phone and computer - Back Door ) calls from family ATAs connected to VoXaLot. i.e. I setup my SIPphone account as a provider in the VoXaLot accounts (yes, more than one) used by my family, and then setup the VoXaLot dial plans to support the free BackDoor dialing. In fact, I even setup the VoXaLot dial plan (on one family member's VoIP phone) to "hard code" a handful of common numbers (that I previously confirmed work with Backdoor Dialing) so that they always take "the free route" by default even if/when the special code to engage "Backdoor Dialing" wasn't dialed with those numbers.