Massive price change despite agreeing and receiving 18-month contract with them in March 25.

I agreed with retention at the end of Feb for a new 18month contract for the same services I had for the last 12 years (with upgrades the same but obviously a few tweaks. I got the pre contract then the contract through and started paying the agreed rates. Then two months later after paying the agreed rate I received an online Bill for nearly three times my agreed rate. After speaking to retentions today and then being called by a supervisor as a complaint was raised, they rang me back to tell me they no were sending me to customer relations on a transfer call, and they would sort out as they couldn't honour the original contract because something g had messed up on the offered and accepted prices. There was no correspondence of any time before this big bill came in and my contract had clearly laid out dates up to October 2026 and all matched with what was being asked to pay on my phone call, and I was happy. Not happy now and still on hold as they seem to have transferred to the holding line telling me it is busy - after telling me there was no wait time. Absolute disgrace