Like that bo bian la. Go and appeal first, as wat tamago advise.
Then see wat they reply. 
Also if all cannot, ask Singtel for a installment payment scheme
(call 1688 customer service) Dun wait till the lawyer's letter. By
then all will be too late.
That's wat happened to my $600+ phone bill. Line cancelled, but
still make payment every month. Singtel refer my case to their
lawyer. I ask the lawyer for installment plan (my balance is abt
300). The lawyer proposed that the money to be paid in 3 months,
including an additional $50 every month for their admin till the
account is cleared.
So dun wait till your account goes to the lawyer. They only suck
more money from you. 