Call forward to PSTN on Asterisk + Astbill

A VOIP service using Asterisk + Astbill billing system. Would like to add feature : "Call Forward to PSTN". The operation : a) User-A configured "Call Fwd to PSTN" (eg, no answer forward - 5 rings) b) User-B VoIP call to User-A's VoIP account, c) If User-A not online, the call will forward to User-A pre-set PSTN number d) User-A PSTN number phone ring, answered and talks with User-B This feature should list on member's admin panel and edit options by members, which options may list : 1.) Forward to : (PSTN number) 2.) Forward condition : - Disable - Unconditional (immediately) - No answer forward (include not registered) options for NAF -- Ring time select (1-10) Condition : 1.) Billing cost should charge to the account owner (User-A), rate using existing system rateplan. 2.) Credit=0 or <0 cannot be dial/forward. 3.) CDR may get two records, one is VoIP-to-VoIP call, another is VoIP-to-PSTN call (billing). 4.) Service provider should response all level including update and modification on server side modules and database for this features if need. 5.) New scripts or modification of existing scripts will not affect and make any change to operation, billing & CDR record or any other things to existing system. We provide test-server environments for developing need. After all testing is passed on test-server, service provider should support the buyer to complete and success implements the modules onto the live server with details instructions. Payment will place on escrow account.