u/Equivalent-Garage-33

I run a small business with a Grandlink DP752 base and DP722 handset in the office, and a Grandlink WP820 at the home office. There is one phone number/account. I have been advised to set up as parallel so both ring at the same time, however when they're both connected only the WP820 rings, is there any settings you recommend for me to fix this?