Well written, I was actually having the same issues but it was related to Codec negotiation, so my PSTN site is using a CP-7961 and noticed that my Dial-peer on the PSTN router was using ilbc, and this codec is not supported on these phones, http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/7962g_7961g_7961g-ge_7942g_7941g_7941g-ge/8_0/english/administration/guide/62adm80/62614241trb.html
About the Author:
Andres Sarmiento, CCIE # 53520 (Collaboration)
With more than 13 years of experience, Andres is specialized in the Unified Communications and Collaboration technologies. Consulted for several companies in South Florida, also Financial Institutions on behalf of Cisco Systems. Andres has been involved in high-profile implementations including Cisco technologies; such as Data Center, UC & Collaboration, Contact Center Express, Routing & Switching, Security and Hosted IPT Service provider infrastructures.