Though I don't know about the VoIP solutions, with a service like Grasshopper it doesn't look like you can use your existing number.
You need to understand that you are going to need at least two lines at your church in order to use an on premises system. If you are willing to install another line then there are many inexpensive solutions available to you. The one exception is if you can still get Centrex service from your phone company. Then you only need one line and an auto attendant like the Partner ACS+Partner Messaging (available on the refurbished market).
-Hal
