VoIP can be tricky - stay away from going exclusively VoIP, for example using Vonage, Broadvoice etc... for business in my experience it's just not there yet. The trickiest part will most likely be choosing the right phones and integrating with whichever PBX / Gateway you'll be implementing. Asterisk is a very solid option - but make sure the server that it's running on is reliable and that the IRQ issues aren't a concern with the hardware.

Getting outbound VOIP Lines might not be mature enough for your company yet. There are always call quality issues unless you manage to get physically near your termination provider and you have a fat pipe from your offices.

The fact that you're a part of a development house is going to help out a lot when customizing your solution. Asterisk really isn't that complicated - modifying it so that it fits your companies needs and provides true business benefit is probably the biggest thing. (Like integrating it with your existing CRM solution or back ending VoIP to your database).

There is a PDF which helps on the overall analysis and how Asterisk can be pretty usefull for smaller businesses =
A Voip Small to Medium Business Analysis