Hi Spaz,
For completed phone calls, we cannot set connection to them.
So we have to new a campaign lookup field and use a business rule.
Step 1. new a lookup field of campaign

Step 2. add this field to the phone call form

Step 3. new a business rule


IF
Activity Status equals "Completed"
THEN
Unlock Campaign
Save and activate it.
In this way, we can select the campaign for the completed phone call, and when you close the phone call form, it will save automatically.