VoiceHost offer free call waiting, which allows you to see who is calling while you are already on a call (dependant on hard/soft phone). Since VoiceHost also offer a free call hold service you can place the first call on hold and greet the second caller.

With a Caller ID enabled phone you can see the second caller's details displayed on screen, so you can decide to answer the call or let the caller be passed onto the free voicemail service.

The call waiting service is enabled by default, all you need is a hardware/software phone that is capable of call waiting indication.

Most phones support this feature, either by displaying the callers information on screen or by playing a tone when a second call is received.