Target any organisation type
There are two options when deciding how to let users pick specific target organisations in phone or peer to peer conversation campaigns:
Default: pick a target organisation that is within the same organisation as the user.
Volunteer anywhere: pick any target organisation in the campaign.
For organisations with a more than one branch in their organisation structure, the 'pick any organisation' option allows you to target by organisations in another part of the organisational structure.
For example if you have this structure, without allowing users to volunteer anywhere a national user can target all organisation types, a regional user can target branches but not employers or workplaces:

So to allow a regional or branch user to pick targets from employers and workplaces, you need to use two settings to enable volunteering anywhere:
Volunteer anywhere permission on user role:
For any user you want to be able to target outside of their position in the org struture, they need to be assigned a role with the volunteer anywhere permission enabled:
Campaign setting "Volunteer targeting" set to "any organisation":
For any campaign you want to allow people to target organisations outside of their position in the org structure, select "Any organisation" in the Volunteer targeting campaign setting.