asterisk based voip-fax-billing solution-saas-web based

Repeat as another project _ Our client needs a Turnkey inexpensive solution. Their website is up, but need the backend as described below. Just Like: RingCentral(.com) and Grasshopper(.com) Client looking a voip solution that can be installed in a CENTOS VPS, where I can offer Voip subscriptions, Virtual Fax and includes a billing system (with gateway integration like paypal for recurring billing). The voip handler must allow me to set PBX and autoattendants for my clients. And of course a small control panel for my client where they can set up basic functions as voicemail, forwards, e-911 address changes, virtual fax, etc. I have my Voip minutes and DID provider. If you have samples already working online, please, let me see or test those projects. I am web designer and know HTML and design programs, so I can do the design customizations. I've heard asterisk can handle some of these features, BUT I dont know Asterisk installations, Telephony techniques, etc. I'm in Need to expand my services into the VOIP area, so looking for an already FRIENDLY (Free-Open) platform that someone has designed and or installed who can offer me the same concept according to what I have described in this ad. Thanks.