Some form of call center capability is often available, though many times either as a different product version or simply a higher pricing tier, so be careful before you assume you'll be getting those features. These capabilities are meant to support large sales or service desk staff and their need to route and process a relatively large number of incoming customer or user calls. That means complex menu trees, an auto-attendant for routing, and service queues. You'll probably find you need interactive voice response (IVR) capabilities, and that should be backed up by support for a live operator or some other type of human intervention.
Mobile clients are softphones optimized for a particular mobile OS and for being used in mobile situations. This means they're designed to switch easily between different cell and wireless connections on the fly. This means you can let your employees use whatever the cheapest wireless connection around them happens to be—and often that can be free. They also let your employees use your company's phone system on their own devices.
First, there might be a very low cost or even free "basic" or "introductory" tier that's just so feature poor that the vast majority of customers will opt for the next level up, which will be the full-priced tier. Another common practice is a one- or two-year contract, each with a slightly lower price that are offered next to a significantly higher-priced month-to-month tier. Additionally, while most residential VoIP services offer unlimited calling, some vary their pricing on call restrictions. Those will come either in the form of minutes (with higher pricing attached to monthly overages) or geographic region. The latter usually start with nationwide calling and then tack on another charge for worldwide calling or even separate charges for different countries.
Typically, price is one of the most important reasons people opt for residential VoIP. One of the most attractive is the "triple play" sales pitch we mentioned above made by almost every regional residential cable company and internet provider: Get your Internet, TV, and phone service all rolled into one monthly charge. Not only is that usually an attractive number, it also means a technician will hook everything up for you including your phone, and you'll probably be able to use the same phone you're using now instead of having to migrate to a VoIP phone.