Incoming calls being blocked may not be Maxis' fault. My wife's Hong Kong SIM card, which is 3G, doesn't receive calls either when used on iPhone. I have 2 iPhones here in Penang. We played around with the SIM cards from Maxis, Digi, CSL from Hong Kong. Only Digi can fully utilise all the functions of the iPhone, except the visual voicemail. We switched cards between iPhones, a Nokia and a Razr. Since iPhone is a 2.5G phone only, maybe 3G services do play havoc with the iPhone. Don't blame Maxis. Customers of Maxis can receive calls if they don't use iPhones to do it.
Anyway, I use Digi, so it works for me. Unless I can find a Maxis SIM with no 3G activated yet, it just might work. I'll keep you guys posted.

If you use DIGI network,to get EDGE to work on iPhone, , go to Settings/General/Network/EDGE - . You will see APN , Username and Password. Key in APN to - diginet. Username and password is - digi