you can do this on the admin portal and on your iphone.

ON THE MOBILE APP
click on your profile picture
select Phone
select Call rules & voicemail
select Incoming calls
do the Ring settings - you can configure if they should ring simultaneously or sequentially. You can toggle either the app or the deskphone ON and OFF. You can configure how long they ring for (select "Ring all together" or "Ring in order" to configure the duration of ringing)

ON THE ADMIN PORTAL

Users
Click on your user
select Phone
select Call rules & voicemail
select Incoming calls