Rang twice, I answered but no one spoke

Caller: No idea who is calling

This number has called a few times but no one talks when I answer they just hang up.

I forgot to add in my other post on (03) 7053-390X that I have a VoIP landline and the calls come through the modem which is being attacked.

I get many nuisance calls from 03 7053 390X. Only rings 2 -3 times then stops, if answered in time, silence or dial tone. These calls all coincide precisely with alerts in my modem's system log eg
"Apr 3 11:22:40 user alert kernel: Intrusion -> IN=ppp0.1 OUT= MAC= SRC=206.168.34.166 DST=100.71.183.88 LEN=60 TOS=0x00 PREC=0x00 TTL=52 ID=27782 PROTO=TCP SPT=46672 DPT=60948 WINDOW=42340 RES=0x00 SYN URGP=0 MARK=0x8000000"
NetComm and my ISP say these are attempted intrusions being blocked by the modem's firewall.