You can have a virtual PABX and use your mobiles as extensions in this one. For that, it will be enough to install a softphone (Zoiper, Media5fone, Bria, GS Wave and many others) in each mobile and configure the extensions of the virtual switchboard there.

If there are phones without Internet coverage, you can forward calls to the mobile phone number without having to install the softphone. When you transfer calls to a mobile phone without Internet, the call is received in the SIM card number. You can also use an access number to make calls with the virtual switchboard's header number.