With offering Voip service to small and medium businesses, here are few things to keep in mind.
1. Many of these business may already have a traditional PBX or key system.
In this case, you made be better to sell them on the service offering a VoIP to T1/PRI/Analog PSTN gatway onsite.
This would allow them to keep the same phones and pbx, same IVR and extensions settings, etc. as they already have configured.
You would just be replacing the T1/PRI/Analog input they currently purchase from their local telco.
Some of these devices can be found here: http://www.voipsupply.com/ip-gateways
2. They do not have an existing setup or are willing to trash their old system completely.
In this case, you can go a couple ways:
a. hosted pbx - you would host an asterisk or some other PBX at your location or colocation facilities, and provide them IP Phones that would connect to your system.
In this scenario, it would cost the customer less up front, because all they need are the phones, and you can charge the monthly recurring for them to access your pbx.
General IP phone varieties: http://www.voipsupply.com/ip-phones
b. Onsite PBX and phones - Customer could host a asterisk/trixbox/or voip appliance, and connect to a softswitch of yours for service, their phones would connect locally to their pbx onsite.
some voip systems that could be used onsite: http://www.voipsupply.com/phone-systems
Most of these solutions will allow for Dynamic environments utilizing Username/Password registration/authentication.
The voip-info.org site also has many resources for researching different pbx and softswitch companies:
and Voip billing solutions:
I have seen many people starting out use A2Billing as a free solution that works with asterisk.