Hi and welcome to the site, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially, please click HERE where the manual may be viewed / downloaded from.
The manual is in a Portable Document Format (PDF) file and can be viewed in most web browser. Click on the link to open the file or right click then select “Save As…” to download the file. It would require Adobe Acrobat Reader or equivalent for this purpose.
See p. 41 for remote operation and the remote access code.
Basically, call your home phone/Panasonic KX-TG6644B's number, using the touch-tone keys of the calling phone, enter the 3-digit access code and follow the prompts.