Nowadays, voice over IP (VoIP) telephony networks are connected to classic public switched telephony networks (PSTNs). Emergency calls from VoIP peers to PSTN public service answering points (PSAPs) are possible. Through the connection of IP networks and PSTNs the PSAP may be a victim of new, more powerful denial of service (DoS) attacks. This paper describes the present and future architecture of a PSAP. Based on measurements at a PSAP the challenge of attack detection at the PSAP is revealed. Furthermore, first solutions are pointed out and evaluated.