- Oct 9, 2002
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Talked to someone in a different department at my employer. They sell IP phone service to local commercial customers. Previously, they've been using Cisco Matrix phones that use "Skinny" (SCCP) protocol, but they've recently started using GrandStream IP phones that use SIP. Multiple users have reported getting phone calls from strange numbers(like 100, 1000). The line is always dead when they try to answer. We checked with the company that enables the phone service for us, and they said these incoming calls with the provided timestamps did not pass through their voice switch, so it must be a device-to-device call using SIP.
I told the commercial phone department that doesn't make sense to me. Customers are experiencing this behind a NAT router. Their phones do not have public Internet-accessible IP addresses, and it's unlikely that the router is configured to forward incoming connections from the Internet. It seems to imply that another device on the LAN is contacting the phones. How likely is it a LAN computer is compromised and making strange IP calls to SIP phones on the LAN?
I told the commercial phone department that doesn't make sense to me. Customers are experiencing this behind a NAT router. Their phones do not have public Internet-accessible IP addresses, and it's unlikely that the router is configured to forward incoming connections from the Internet. It seems to imply that another device on the LAN is contacting the phones. How likely is it a LAN computer is compromised and making strange IP calls to SIP phones on the LAN?
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