You can hear the American call-waiting blip when an inbound call is waiting and to change lines you need to "Flash" the line, which means hang-up for about 1/4 of a second and then the SPA-3000 will switch lines. If you flash the line again it should go back to your first caller.

If you are still unable to switch between the calls you may need to adjust the hook flash timer.

The "Hook Flash Timer Min:" and "Hook Flash Timer Max:" parameters on the Regional page control the hookflash timer. The SPA-3000 default settings specify that flash event must be between 0.1 seconds and 0.9 seconds.

Some users report a hook Flash event works well between 0.4 seconds and 1.4 seconds.