VoIP is a subscription-based service, and you usually have the option to pay monthly or annually. With Dialpad and most service providers, you save more by opting for an annual plan instead of a month-to-month plan. Since then, many technologies have changed. Nowadays, you don't need a voip PBX at all if you subscribe to a cloud-based voip phone system.
In fact, if you have a data plan on your mobile phone, you can make and receive calls from anywhere, anytime. With Telzio, you don't have to pay for users who rarely use the phone. At the same time, you can give everyone a phone line to use. Much of the documentation available on the web states that you need both the VOIP PBX AND a subscription to a voip provider (for a VOIP phone number).
Because VoIP services are often subscription-based, you'll be charged a regular fee until you decide to cancel.