IPkall number forwarding to VOIPtalk not working - always busy signal

I hope someone can help with the following:

I got a US phone number from IPkall.
I set up an account with voiptalk.org, I have heard that it's possible to forward an IPkall number with a router to a voiptalk.org account.

I am using a FritzBox router and set my voiptalk.org account details up as
instructed in the user guide from voiptalk:

-other service provider
-ID and number 844++++++
-password my voiptalk password ++++++
and the connection is showing as fully registered, and online in my router.

With IPkall I also set it up at their end as instructed,
with my 844++++++ no as the phone number.
[Also ones tried the above settings - with the given voip phone no, i.e. with the 11 digit long number provided by voiptalk.org instead as the sip phone number, but that didnt' work either, nor does voiptalk.org mention this in their set up guide.)

However when testing the IPkall number getting transferred to my set up
voiptalk number in my FritzBox I am always only getting a busy/engaged US phone signal.
(N.B. The test is done from a different phone, using a landline, so the
busy signal is not because I am using the phone line myself.
Also the setting of the above, hasn't just been done, but it's now a week ago, since I set it up, and it's still the same busy line only.)

I hope someone can advise, what I am doing wrong.

Thanks.