consultance for asterisk a2billing voip platform

Customized elastix a2billing design or openser (kamailio)? The main idea is that several customers are going to use our asterisk based PBX. For each "slave" PBX all of internal users are going to have access to international phone calls and their PBX it is going to be link to our softswitch via IAX trunking due to DNS bug in Asterisk (if customer looses the internet connectivity all internal SIP phones are going to be dead one ore more SIP trunk not to be found , DNS resolution in chan_sip.c it is going to freeze up the system )? I need help in elaborating the specifications for a upcoming project. The coder is going to help me out with a network diagram , project specs , and the project will be reloaded to rentacoder for design/implementation? All the ideas are going to be presented to the wining bidder? The final recommended design it is going to be based on opensource software , easy to implement and using robust products on the market with possible custom interfaces API (asterisk, a2billing , elastix , kamilio)