Where do we start?

Good day to all!
Im new to this forum and Im a new to VoIP, I have only little exposure on VoIP set-ups so please bear with me,

We are planning to set-up VoIP to our company, its a chain of stores and hotels plus the offices, linked by backhaul radios, a total of 11 sites, and maybe more or less 150 locals, our main concern for now is the local calls to each site, maybe the in and out going calls will come later,

So where should we start on our design,
What should we consider on our designs.
Any recommendations of sotware or hardware.

Any inputs are very much welcome!

Thanks and more power!

How large an area are you covering and what is between the locations?

Hotels and shops suggest quite a few acres, if not square miles, possibly with other shops, offices and hotels in between. Which would make me look at the radio network first. I am guessing it is not a simple low-power 802.11 WiFi installation?
Does it give clean high speed data transfer at all points, not just where coverage is required but between stations as well?
A few lost packets and/or low transfer speeds because some peasant is stood between nodes is not a problem to most data applications, but you will notice them with VoIP.
Security I'm sure you already have in hand.

After that, it sounds as if it will be quite a low load in terms of calls, so any of the pre-packaged Asterisk solutions (AsteriskNow, Trixbox etc.) will do the PBX job

The wireless link that we have right now is pretty much stable, we uses some backhaul radios thats running on a licensed band, latency is pretty much less and packet lost are very rare,

In that case give an Asterisk distribution a go, at least as a test bed.

If it doesn't work it won't have cost anything and you will know the whole project is difficult enough to need a specialist.

If it does, then just keep expanding.