What do I do??

Has he now told you who this woman is and what the call was about? Perhaps the call really wasn't a big deal, but if he can't tell you either of those details (or, hell, didn't volunteer the info) that's a major red flag for me.
This is a vulnerable time for you and he should especially be respecting that right now, even if he feels you've jumped the gun a bit. The fact that his first instinct was not to reassure you is the most worrying bit for me.

Yeah he can’t tell me who she is and said the call was a catch up…

Who is this lady? And why is he texting her?

I have no idea, I’ve never heard of her before. She even flagged to him in the messages that she should tone down the flirting as she knew he has a partner, but he said why would you tone it down and then called her late last night for over 20 mins

Tbh the best thing for you to do is leave bc once trust is broken it’s very very hard to get back, you have to have a long talk with yourself do you want this partner who clearly doesn’t respect you enough or do you want to constantly feel like you’re second choice in a relationship where you should’ve been first choice? Trust me with my ex I had to have that talk to myself and it wasn’t easy and it hurt a lot but I’m so so glad I got out bc if I would’ve stayed I wouldn’t have my life that I have now and it will be hard at first lord knows it will be but in the hard times think about your baby(s) bc the last thing you want is for your children to see this is how love is. Keep your head up and remember sometimes the biggest hurt is at home and the best thing to do is leave the hurt before it consumes you and it’s all you can think about. If you need to talk you are more than welcome to pm me