Yes, ATRG: Gaia Embedded Appliances is surely the best starting point ! For people without much experience with VoIP, i would suggest to read sk95369 ATRG: VoIP for CP implementation details. Apart from the Admin Guides, you find in sk113573 Configuring VoIP on Locally Managed 600 / 700 / 1100 / 1200R / 1400 appliances further information. VoIP Debug on SMB is covered in sk108815 Basic VoIP debugging when phones located behind firewall and PBX is external.
Further reading:
sk105981 PBX and SIP phones are unable to register on Locally Managed 600 / 1100 / 1200R appliance
sk108275 How to configure VoIP SIP when PBX is located in LAN network of Locally Managed 600 / 1100 / 1200R appliance
sk109403 "Value already exists" error is displayed when editing SIP_UDP or SIP_TCP services
sk111532: SIP Keep-Alive Messages on locally managed SMB Appliances
sk108815: Basic VoIP debugging when phones located behind firewall and PBX is external
CCSP - CCSE / CCTE / CTPS / CCME / CCSM Elite / SMB Specialist