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When sending the UDP packets via SER
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1) When sending a CANCEL with a changed tag (within the From field) it is ignored, however the second CANCEL with the right tag does not cancel the call
2) When trying to put a random string in the branch parameter (within the Via field), the cancel does not interrupt the call, even if the branch, tag, and call-id are the same (branch must be something like the following branch=z9hG4bKfae8cb69f547b8cb). According to RFC 3261 the branch identifies the transaction [p.12]

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When sending the UDP packets directly to the SIP phone
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1) When sending a CANCEL with changed tag it will be ignored, but another CANCEL with the right tag will cancel the call
2) Branch parameter has no impact on the association of the INVITE with CANCEL. The branch parameter can be different in two packets, but the call will be canceled if the tag and call-id are matching
3) For a SIP phone it does not matter if the SIP messages contain correct R-URI in the first line of the text encoded message following the method name and in the To field.

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In both cases (via SER or Directly)
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1) When sending a CANCEL with changed call-id the message is ignored; another CANCEL with the right call-id cancels the call