Yes, maybe on a bigger perspective, is there a way to select what type of objects actually show on the messaging calendar?

Long story short, to see the crm_phonecall in the calendar you will have to inherit from calendar_event and reimplement the create method in order to add the fields from calendar_event. Probably you will want to reimplement the write method as well, but that is really up to you.

In your module:

class crm_phonecall(base_state, osv.osv):
    _inherit= "crm.phonecall, calendar.event"

    def create(self, cr, uid, vals, context=None):
        ...
        # add required fields for calendar_event to vals
        ...
        ret = super(crm_phonecall, self).create(cr, uid, vals, context=context)
        return ret

Hope it helps, although it has been a while...

You could install

openerp.com/apps/7.0/planning_management_shared_calendar

The result is by default in menu Project/Calendar/Shared Calendar

You can ask him to merge meeting calendar and phone calendar in this new calendar every minute.