That could just be a temporary glitch, but it could also be more complicated.
It sounds like the phone you're calling might have its slot cycle setting a bit too high.
The slot cycle is the interval between polling of the nearest tower for incoming calls. The higher the number, the longer the interval, so a few rings can go by without the receiving device even knowing they are there.
The default setting on some devices is not compatible with the towers (that's why the problem will often be only in certain areas for that particular phone), it's usually at "2," which is between two and three seconds to poll. Setting it at "1" or "0" will get the calls to the device quicker, but battery life suffers a bit because of the increased activity by the phone looking for calls.
Each device/carrier has its own code (accessed by the phone dialer) to get to the menu for changing the slot cycle, sometimes it can be done in the store, but only if there is somebody there competent enough to know all this and about that specific device.
