Cannot answer call for some time

[AC Question]

When someone calls me on my galaxy s4 I can hear the ringing but I cannot see the usual answer screen for at least 15 secs after the ringing has started. This means that I cannot answer the call for 15 secs. Although this doesn't sound like a long time, it is a little frustrating to be sitting there looking at a black screen waiting to be able to answer a call.