For escalation step that uses fixed group, when on_call runs then it is not sending notification to this group. It sends notifications to individual users when mentioned, but not to the group.
If there is no shift defined for the group, no email will be send.So there will not be any escalation for that group even if you add it in the escalation steps.
Also, when we say escalation to the group(which is defined in the escalation step), If we are expecting that system will send an email to that group (i.e. members) since it an escalation by email workflow , then in this case, system will trigger another escalation workflow for that group .
More can be reach here on escalation by email :