Calling from overseas? 0800 is a toll free number within Brazil. Whatever service you use for international calling may be able to tell you if you can call such a number (obviously not toll free). Or perhaps you can find a non-0800 number.

I have always found TAM's prices nearly identical to those of Gol and never had trouble booking through their site, but have only booked through their Brazil site. Last time I flew them (FOR-GIG-FOR), I got a better price going through Expedia.com, which had the added advantage of allowing me to pay in USD and not have to pay my credit card company's 2.7% foreign currency transaction charge. Expedia did not show any Gol flights (except as pat of international itineraries involving other airlines). When I booked through Expedia, I was even able to cancel a flight over 24 hrs. after booking (my wife had to change her flight plans). I imagine you could also book through other sites, such as Travelocity and Kayak. There may well be third party sites that book Brazil domestic flights on Gol; I just haven't seen any. Personally, I would have no particular preference in flying Gol vs. TAM. Haven't yet flown Webjet, Azul or Avianca.

Avianca Brasil (connected to, but not actually part of, Avainca International based in Colombia, at least not yet) seems to have lower prices on some routes, though, of course, far fewer flights than Gol or TAM. They seem to be using Fokkers (vs. Boeings for Gol and Airbus for TAM). I haven't booked a flight through them yet, but did look into it and their site appears to offer options to use either a Brazilian CPF or foreign passport number in order to register.

As of last weekend, at least, Webjet was still flying as a separate airline (we saw several Webjet planes when taking a friend to catch a flight at FOR).

I am currently hoping for a sale on TAM or Gol this weekend for a flight to Brasilia and back next month. Prices jumped over the past several days.