No service provider offers fully encrypted calls via the PSTN, but if you wish you may set up a VPN between two or more locations, with an OBi at each end, and make encrypted calls over your VPN.
Unless you're discussing trade secrets worth $X,XXX,XXX or more, this is likely more trouble than it's worth. Keep your home network locked down tight (change default passwords, use WPA2 with a strong key, don't use DMZ or port forwarding, use antivirus software, don't leave your network physically accessible to the public) and call it a day.