OK, an ADSL line is an old fashioned POTS analogue line.
So, there are two potential problems I can see that might stop you
1) You are NOT using a filter on the line for the dialup modem (it needs to use the same microfilter as your analogue phone, otherwise you will have noise on the line)
2) Are you using a dial-up username and password or the ADSL user name and password (Telkom allows this I think?). Most ISPS will require a separate username and password for dialup.