The pagewrapper can't be selected from the Call Alert configuration page, but it can if you edit the Action Alert that corresponds with the Call Alert. It can be edited by doing a URL hack.
  1. Advocacy > select Call Alerts
  2. Find your Call Alert and click Edit
  3. Look at the URL in your address bar and make note of the number after id=
    • Example: https://secure2.convio.net/shortname/admin/AdvocacyAdmin?id=149&page=ActionAlertInfo&cmd=display
  4. Advocacy > Action Alerts
  5. Edit any Action Alert, it doesn't matter which one you choose
  6. Replace the number after alert_id= with the number you copied from step 3 and hit Enter
  7. Skip to step 9. Design Alert
  8. Select the pagewrapper for your Call Alert on option 1. Page Wrapper
Alternatively, if you are going to be sharing the link to your Call Alert, you can specify the pagewrapper in your URL with the pw_id parameter. For example:

https://secure2.convio.net/shortname/site/Advocacy?alertId=149&pg=makeACall&pw_id=1111

Where 1111 is the ID of the pagewrapper.