There are a few issues you might want to check. l use to have similar issues as well, line dropping, slow speed etc and singtel have to get a techician to check from the local exchange to my flat.
Some potiential problem
1. The telephone line condition. Is your telephone line very old or did you check if there is any oxidisation on your line, i.e. those green color dirt on your copper joint. High humidity is the cause and u might need to clean it frequently, else signal strength will weak and there will be a lot of noise.
2. U are using a 2 line or 4 line connection?
3. Are u using any splitter or joint? more splitter/joint means more signal lost.
After l know the problem and do the necessary regular maintenace, don't really have serious problem until now.
Of course if the issue is at the local exchange, than is beyond your control and Singtel need to do something.
Good luckl.