Both your cell phone and smart key are transmitting using Bluetooth at all times, but is incredibly unlikely they are interfering....

Bluetooth communicates at a frequency of 2.4GHz. This is the same frequnecy as baby monitors, some cordless phones, and 802.11 wireless internet. In order to avoid interference, Bluetooth uses frequency hoping. Each bluetooth device has 1 MHz of bandwidth for transmission, and that 1 MHz jumps around the 2.4GHz band 1600 times per second. There are 79 different frequency positions. This hoping is pseudo-random. If two devices are sharing the same frequency, it will only be for 625 micro-seconds.

What does all this mean? It means that if your cell phone and your smart key happened to transmit on the same frequency, it would be for less than 1/1000 second. The two devices would then move to different frequencies.

I would check the battery on your smart key, and also see if there are other things can could be interfering, such as 802.11 networks, baby monitors, cordless phones, etc.

I hope this helps!