Is there a way to disallow all outside calls and

Check your telephone to see if it has an intercom feature. Many wireless phones that come with multiple units allow you to temporarily turn one unit into an intercom to contact other units attached to the same wireless phone system. Depending on the model, you can contact all the other phones at once, or a single specific phone, by pushing the intercom button and entering the other mode. To put the phone back into regular mode, all you have to do is switch off the intercom option.

Contact your telephone provider. Some telephone service providers allow landline phones to call themselves, which enables the phone to function as an improvised intercom. You would simply pick up the phone, dial and then close the connection. Once the phone is answered elsewhere in the house, you will have established a line to another phone and you can speak normally on it. This has largely been eliminated due to lack of need as technology advances, but it may still be an option with some providers.

Purchase and install an intercom system that will work through an existing phone line. Many of these products, some of which are produced by Panasonic, operate as a halfway point between a traditional intercom system and a phone line. The exterior intercom links directly to the phone line, and you can pick up the phone and talk through the intercom unit outside once the system is properly set up