This probably means that your phone hasn't been setup to "loop" ringtones. In that case, if you don't answer the call within 30 to 50 seconds, and your network doesn't terminate the call request before then, the ringtone will end.
NB 30 to 50 seconds is a long time in terms of answering a call, and the network operators do not let an unanswered call ring out for long, because that ties up network capacity without generating revenue.