So she had a monthly credit limit of £40 put on 12-year-old Bethany's mobile which she bought at The Carphone Warehouse.
The 35-year-old customer service adviser said she was told by staff at the firm's White Rose Centre store at Churwell, Leeds, that the phone would cut out when the limit was reached.
But a month later she was stunned when a bill for £310.39 dropped through the letterbox.
Miss Whiteley, of Beeston, Leeds, is now demanding the retail giant cover the cost of the calls and texts over the credit limit.
She said: "At the time I was confident that the phone would cut off when it reached the limit. I never thought for one minute that she would be able run up such a huge bill. I was really upset and angry when the bill came through."
Miss Whiteley said the 12-month contract she signed in store made it clear that there was a £40 credit limit on the phone for calls and texting.
"The salesperson in the shop even underlined it for me. I would not have signed up if I knew there wasn't a credit limit."
Miss Whiteley telephoned Carphone Warehouse. She said: "They have offered to pay half of the bill after the £40. That is not good enough."
Miss Whiteley has also taken up her complaint with telecoms regulator Oftel.
Liable
An Oftel spokesman said: "Our consumer team will be working with the mobile retailer involved to resolve the problem."
A spokeswoman for The Carphone Warehouse confirmed that upon signing her contract the firm's Leeds store underlined the various details of the tariff.
"This included details of the customer's call limit. However, the limit itself is designed as a guide to monthly call spending and does not automatically restrict phone use when reached.
"As our terms and conditions point out, the call limit can be breached because the network systems do not work in real time and there can be a delay in call data reaching the service provider. Customers are therefore liable for all call charges incurred including any charges exceeding a usage limit.
"When a call limit is breached, we place a bar on the line that prevents outgoing calls being made. However incoming calls and text messages are not affected. Only once a 30-day period of non-payment has elapsed will our Credit Control Department consider administrating a full bar.
"To completely bar a line and text messages in the first instance would necessitate a network reconnection charge and may only serve to compound an already delicate situation."
The spokeswoman said records showed 1,359 text messages had been sent after a warning text message that she had reached her call limit had been sent.
"We do apologise however to Miss Whiteley if she feels that any of the above was unclear and as gesture of goodwill we would like to offer to pay 50 per cent of the outstanding charges."