If your office or home phone works via SIP, you can use that phone number on your Mac anywhere you have decent Internet connection. The prices are displayed in Telephone’s storefront at the time of purchase.
Telephone is a VoIP program which allows you to make phone calls over the internet. Telephone Pro is a subscription that unlocks the full call history, allows 30 simultaneous calls, and supports ongoing app development. It can be used to call regular phones via any appropriate SIP provider. The subscription has a monthly and a yearly option.