A security device for SIP communications operates to inhibit the effect of malicious attacks and/or inadvertent erroneous events on the provision of SIP-based services within a private network and between private and public networks. The security device acts as a conventional Firewall, NAT and PAT to isolate SIP User Agents on the private network from SIP User Agents on the public network and to Blacklist undesired callers. Also, the security device preferably includes a virus scanner to scan attachments to sessions and/or other communications to identify and block virus contaminated data and the security device includes a hardened SIP stack to scan for and detect malformed SIP messages to prevent malicious attacks and/or inadvertent erroneous messages from adversely impacting the operation of SIP services.