16 week midwife appointment ?

Hi all I’m currently 17weeks1day and I was told I need to book in my midwife appointment at 16 weeks which I haven’t done as I don’t have their number and not heard from them at all in this pregnancy I saw one at 9 weeks and that was it the only option is calling my gp but I can’t get through to my doctors as they are only taking emergency calls today.

What happens at this appointment?

Is it necessary ?

UPDATE: just to say that I’ve tried calling the community midwifes with the number in my book many times this morning with no answer so I tried again this afternoon and still no answer so will try again tomorrow with both gp / community midwifes.