Do I need to have a specific plan on my SIM card in order to use it with the Onoff Business application?

As your Onoff number is directly linked to the sim card installed on the mobile used, your calls are therefore deducted from your package under the same conditions as the native number.
If you have an unlimited call plan, then your calls from your Onoff number will also be unlimited.
If the operator charges for calls to or from abroad, the same applies to Onoff calls.