Riporto il tcpdump fatto direttamente sull'interfaccia voip, senza nessun filtro impostato:
21:11:42.906744 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 106: PPPoE [ses 0x599d] IP (0x0021), length 86: 1.255.160.20 > 10.174.0.32: ICMP echo request, id 47689, seq 19, length 64
21:11:43.916304 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 106: PPPoE [ses 0x599d] IP (0x0021), length 86: 1.255.160.20 > 10.174.0.32: ICMP echo request, id 47689, seq 20, length 64
21:11:44.927769 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 106: PPPoE [ses 0x599d] IP (0x0021), length 86: 1.255.160.20 > 10.174.0.32: ICMP echo request, id 47689, seq 21, length 64
21:11:45.945239 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 106: PPPoE [ses 0x599d] IP (0x0021), length 86: 1.255.160.20 > 10.174.0.32: ICMP echo request, id 47689, seq 22, length 64
21:11:54.744703 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 30: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Request (0x09), id 54, length 10
21:11:54.759168 e0:24:7f:fa:4e:04 (oui Unknown) > a4:91:b1:34:3d:9b (oui Unknown), ethertype PPPoE S (0x8864), length 56: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Reply (0x0a), id 54, length 10
21:11:54.889661 e0:24:7f:fa:4e:04 (oui Unknown) > a4:91:b1:34:3d:9b (oui Unknown), ethertype PPPoE S (0x8864), length 56: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Request (0x09), id 36, length 10
21:11:54.897470 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 30: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Reply (0x0a), id 36, length 10
21:12:14.771311 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 30: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Request (0x09), id 55, length 10
21:12:14.785776 e0:24:7f:fa:4e:04 (oui Unknown) > a4:91:b1:34:3d:9b (oui Unknown), ethertype PPPoE S (0x8864), length 56: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Reply (0x0a), id 55, length 10
21:12:24.890138 e0:24:7f:fa:4e:04 (oui Unknown) > a4:91:b1:34:3d:9b (oui Unknown), ethertype PPPoE S (0x8864), length 56: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Request (0x09), id 37, length 10
21:12:24.898501 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 30: PPPoE [ses 0x599d] LCP (0xc021), length 10: LCP, Echo-Reply (0x0a), id 37, length 10
21:19:16.791033 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 804: PPPoE [ses 0x599d] IP (0x0021), length 784: 1.255.160.20.sip > 10.174.0.32.sip: SIP: REGISTER sip:proxy1-VP00181-VMCNVS1.proxy.dsl.vodafone.it;user=phone SIP/2.0
21:19:20.800052 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 804: PPPoE [ses 0x599d] IP (0x0021), length 784: 1.255.160.20.sip > 10.174.0.32.sip: SIP: REGISTER sip:proxy1-VP00181-VMCNVS1.proxy.dsl.vodafone.it;user=phone SIP/2.0
21:19:22.789931 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 809: PPPoE [ses 0x599d] IP (0x0021), length 789: 1.255.160.20.sip > 10.207.161.174.sip: SIP: REGISTER sip:proxy1-VP00181-VMCNVS1.proxy.dsl.vodafone.it;user=phone SIP/2.0
21:19:23.292051 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 809: PPPoE [ses 0x599d] IP (0x0021), length 789: 1.255.160.20.sip > 10.207.161.174.sip: SIP: REGISTER sip:proxy1-VP00181-VMCNVS1.proxy.dsl.vodafone.it;user=phone SIP/2.0
21:19:24.296881 a4:91:b1:34:3d:9b (oui Unknown) > e0:24:7f:fa:4e:04 (oui Unknown), ethertype PPPoE S (0x8864), length 809: PPPoE [ses 0x599d] IP (0x0021), length 789: 1.255.160.20.sip > 10.207.161.174.sip: SIP: REGISTER sip:proxy1-VP00181-VMCNVS1.proxy.dsl.vodafone.it;user=phone SIP/2.
Nessun tipo di reply ai miei ping, ho incollato il resto del dump perché sembra invece esserci passaggio dati slegato da mmpbx (da riga 5 a riga 12), niente di utile comunque, LCP, roba legata alla connessione pppoe messa su immagino