Yea, I couldn't call my girlfriend on her E61 but surprisingly, she could call me and receive my smses.
Met her up and switch it to GSM only. Switched back to Dual mode later on.
One suggestion, if the 3G network is faulty, could Singtel switch (UMTS) it off, that would force handphone to automactically switch to GSM.
It's frustrating that my gf could not receive any incoming calls. What about those who are in their jobs/businesses?
Or perhaps, they can send an SMS to all Singtel 3G subscribers to switch to GSM.