I added an infant to a redemption booking earlier this month. It was on the English speaking line but I do not know which country's call centre unfortunately. I think I just got very lucky as they were able to add the infant on without too much trouble, and did not pass me on to a different department or call me back. I do not think I would have been speaking to a redemption agent as it was around 1pm UK time and so it would have been evening in Singapore, but it could have been I suppose. They took the CC payment details over the phone.

So maybe just trying at a different time of day and hopefully getting through to an agent who knows how to process it is the way to go.

Edit: I did not call a special number - just whatever number was on the website.