Am I able to use 1899 if I am already registered with Alpha or Onetel?
Yes, you can. If you normally dial another Indirect Access code before your calls (for example 1833 for Alpha telecom or 1877 for Onetel) you simply omit this code and just dial 1899 instead.
If you are using a router box (also known as Phone.Pal) you can either unplug the router box or dial 1-2-1 (or whichever override code is provided by your service provider) to bypass your router box followed by 1899.
Should you have chosen to have all your calls automatically routed through another provider (also known as Carrier Pre-Select or CPS) you will still be able to use 1899 as usual. Just sign up and dial 1899 followed by the telephone number to make cheaper calls! Your CPS provider will not charge you for dialling the 1899 code, therefore the 1899 code will not appear on your CPS bill and any calls made using the 1899 code will only be invoiced by 1899.