I have seen different midwives doing things many of the older midwives i have worked with do not call a second but maybe that goes down to experience. Some of the more confident new ones do not call either.

If i am delivering then none of my mentors call a second as there are 2 of us in the room. And there is always a bell.

Also if it is abusy unit and you ahve 4 midwives on duty and you have 4-5 women in labour then they may not be able to come to your assistance anyway.