Openfire + VOIP

Hello, Im new to openfire and im wondering if Openfire can do what i want:

I want to sell user accounts for business and that account has the ability not to just text but also communicate via VOIP to land line phones. I was thinkin on becoming a voip reseller and every account i register i add a phone number to it. I see openfire supports asterisk, so, it is posible to use my reseller voip provider and configure it into my openfire server and offer voip and chat to my customers? if so, do i need any special hardware/software to setup the VOIP, like switchers, gateways?

It is possible to have a customized version of spark with my server address already built in so there is no need to type server address? just username and password.

Thank you