This probably won't help, since telemarketers probably wouldn't call VoIP DIDs, especially if they are doing sketchy stuff like this. However, if you're receiving their calls and happen to be using a SIP phone, this comment is for you.
I don't know of how people would mask their caller ID on a normal land line. Of course people can get those caller ID spoof cards, but where are they calling when they call those numbers? Chances are, it's a VoIP service. If someone who has a SIP phone is called by these telemarketers, they may be able to retrieve their real caller ID.
Unless the telemarketers really know what they're doing, they probably only changed the caller ID field in the SIP header, and didn't touch the P-Asserted-Identity field.
Using asterisk, one can obtain the caller ID out of the P-Asserted-Identity field like this, before having the phone ring of course:
exten => s,1,Set(passertedid=${SIP_HEADER(P-Asserted-Identity)})
exten => s,2,Set(CALLERID(all)=${CUT(passertedid,@,1):5})
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This probably won't help, since telemarketers probably wouldn't call VoIP DIDs, especially if they are doing sketchy stuff like this. However, if you're receiving their calls and happen to be using a SIP phone, this comment is for you. I don't know of how people would mask their caller ID on a normal land line. Of course people can get those caller ID spoof cards, but where are they calling when they call those numbers? Chances are, it's a VoIP service. If someone who has a SIP phone is called by these telemarketers, they may be able to retrieve their real caller ID. Unless the telemarketers really know what they're doing, they probably only changed the caller ID field in the SIP header, and didn't touch the P-Asserted-Identity field. Using asterisk, one can obtain the caller ID out of the P-Asserted-Identity field like this, before having the phone ring of course: exten => s,1,Set(passertedid=${SIP_HEADER(P-Asserted-Identity)}) exten => s,2,Set(CALLERID(all)=${CUT(passertedid,@,1):5})