Dead air when someone tries to call you is usually an indication that your cell phone company's network is trying to find your phone. This can happen when the network expects a response from your phone but there was probably a sudden loss in signal (example: entering an underground subway train). This doesn't happen if you turn your phone off before you lose the signal.
Dead air is common with many cell phone companies. Generally, the person who tries to call you doesn't actually have to keep retrying. When the network does find your phone, that's when the person who is calling you will hear a ringing sound. This process can sometimes take about 30 seconds.
During that process of the network trying to communicate with your phone, the person who is calling you will either hear dead air or fake rings. This completely depends on the network of the receiving person is on. When I say fake rings, perfect examples are the Bell and Telus CDMA networks where the person making the call will hear immediate ringing but that does not necessarily mean that the receiving phone is ringing yet. If the receiving end phone is on Rogers, the outgoing caller will not hear any ringing sounds until the receving phone really is ready to receive the call. I'm almost certain that Mobilicity is set up just like Rogers in that aspect.