No, I am based overseas now and I have already cut off my Singapore credit card but not sure if they can accept overseas credit card payment.
I also heard that singtel charges for whatsapps and want to check if M1 or Starhub do that ?
Since you are out of SG and without a credit card, I am not sure if they accept deduction from your Savings account. If they do, then make your payment there to keep the number alive.
If they do not, then ask your friends/relatives in SG to help - give them your phone number and ask them to top it up for you via those AXS [?] machine in SG.
Top up larger amount [e.g. S$20 instead of S$10] to stretch the expiry date and thus not bothering your friends/relatives too much.
Alternatively, when you are at overseas, you can call Singtel Hotline +65 64822800 to extend your expiry date.
Sorry, don't know about that whatsapps charges, M1 and Starhub.