what does conditional forwarding mean on my mobile phone?

Answer 1

It is a feature that enables your phone to automatically transfer/forward a received call (typically to the voicemail, but also sometimes to another phone number) when a specified conditions are met, e.g. :

- there is no answer after a few rings
- the mobile telephone is switched off
- the mobile phone is out of coverage
- the mobile telephone is busy

There might be more options, depending on the phone and your phone operator.

Answer 2

I'm guessing it's when you are on another call or the phone is switched off - otherwise it rings as normal.