Connecting VOIP to the PSTN and Cellular Networks

Cheapest ATAs and Service - For Calling PSTN Numbers, or receiving phone calls from PSTN Numbers
Configuring GSM VoIP gateways with Cisco Call Manager - Step by step guide
ENUM - Translating E164 numbers to VoIP addresses
Failover Switches - Automatically or Manually Switch PSTN Interfaces on failure for reundancy
FXS-FXO Converters - Convert an FXS interface to an FXO interface
Phone Numbers - Dozens of test and information numbers you can call for free.
Routing calls using a free international calling service - How to use two-step dialing to save on costs.
PSTN Gateways - VOIP to PSTN gateways (also known as: Media Gateways)
VOIP GSM Gateways - VOIP to GSM gateways