My entitlement under UK GDPR

Madcowcj

I am wondering if anyone can advise me on this point. 
Under our mobile phone contract, I tried to transfer my daughter to a zero cap plan, but somehow it went wrong and moved her to an unlimited plan.  I have asked my mobile phone provider for copies of the emails and text messages they sent to me in the last 45 days, because if I had received any message to tell me that the price was going up, I would have queried it. 
They have told me that they are unable to provide these to me due to 'system restrictions'.  My understanding was, that if they had contacted me, they should be able to provide copies and that I can request those copies under UK GDPR rules.
I also asked them to show me what data my daughter had used (because they have charged a flat rate of £50), but all they will give me is a figure - am I entitled to a breakdown of how the data has accumulated?
I would really appreciate any help anyone could provide.  (I am also angry with them, because they tell me they cannot change me to a zero cap until 9th June, which will mean they will charge another flat rate of £50!).

PHK

You can make a DSAR, but this is for information they hold about you. Not for system records, copies of general communications. 

You also aren’t entitled to a breakdown of data. 

Surely you kept your own copies of important communications and have the log on the phone of what data was used. 

I’m puzzled by what you are trying to achieve. Having a zero cap tariff would mean you pay a flat rate each month as you wouldn’t be able to go over your allowance.