Unable to understand why I don’t get a full pension....

And as it is impossible for me to get through to the state Pension helpline I was wondering if anybody on here might be able to throw light on the matter. I went in to my government portal to see what years I had to qualify for a pension and I have 47 years of full contributions and I believe the full pension is £700 a month and I’m not at all sure why l only get £637.92. I do work part time and get taxed on my earnings as well as my pension but if somebody could explain to me I would be most grateful as God knows when I’ll actually I be able to get to speak to somebody at the state pension office!! Thank you in anticipation ?