I wanted the Calls QuickCreate Related To: field to default to Contact but various bugs seems to make this more difficult than it should be.
First: the left box in Related To: ignores the sort order selecting in parent_type_display and always sorts alphabetically it appears
Second: I removed all options from parent_type_display except Contact. When opening a new QuickCreate tab the Related To: field properly only shows Contact but the javascript that tries to autofill/predict as you type will still search Account instead of Contact. If I close/cancel the QuickCreate tab and open another QuickCreate tab it will usually properly search Contact until the page/window/browser tab changes.
SuiteCRM: 7.2.2
Apache: 2.2.22 (Debian)
Debian 7
PHP 5.4.36
MySQL 5.5.41