It's all pretty heavy reading, but:
- Canadian Telecom Information
- ANSI/TIA/EIA standards
- Cisco Telephony Signalling whitepaper library
- Voice Network Signaling and Control - a particularly good resource
Also the United Kingdom (GB) Public Switching Telephone Network Interopabillity Standards, courtesy of 'Anonymous'.
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United Kingdom (GB) Public Switching Telephone Network Interopabillity Standards