My mum has been lying to me. It's either her or Telstra, and I always take Telstra at its word, so it must be Mum.

This month marks the first anniversary of Justice Peter Gray's ruling that Optus offers better value than Telstra. Telstra had tried to stop Optus running an ad stating that its $49 Cap plan for mobile phones was superior in value to Telstra's $40 Phone Plan. In denying that request, Australia's third-most senior Federal Court judge said, in words worthy of any advertorial, "It is undeniable that a consumer would get better value under the Optus $49 Cap plan. Telstra cannot show to the contrary."

I read all of Justice Gray's judgements; he's the Helen Wellings of the federal bench. I assume most Australians read his stuff, given that late last year the number of mobile phones we use topped 20 million, which means that even toddlers are now wandering around with them. Which is why I was surprised to find out that my mother was still on Telstra's $40 plan - the very one Gray had so scathingly criticised.

Mum had asked me to come with her to the Telstra shop to help her get a new phone. Telstra had sent her a "lovely note" urging her to renew her contract on the current plan, for which she would receive $150 in "bonus credits". It didn't mention what bonus credits were, or how they could be redeemed. But $150 of credit did sound like a great deal.

She showed me a copy of her latest monthly bill. It itemised 38 calls lasting between 30 seconds and ten minutes. Most of them lasted no more than one minute. For this, she had been charged a total of $87.33, or about $2.30 per call. What was Mum thinking? Had she not seen Australian Personal Computer, which featured a long report on the implications of Justice Gray's ruling? Had she not heeded Choice's concise 16-point summary of the pros and cons of pre-paid and post-paid mobile-phone plans?

She admitted that she had been surprised by the size of her bill, especially as Telstra seemed to be charging her twice. After charges for each call, an extra $40 was lumped on top for a service with the line item "Phone Plan $40". The "nice man" at the local Telstra shop had assured Mum that it was a standard charge. On the back of the bill, Telstra told her that she had saved a full $7.81 by being on this plan. At least she wasn't paying $95.14 for 38 calls. Then she'd be a sucker.

I asked her who had suggested that she choose this plan two years ago. That nice man at the Telstra shop, she said. It was then that I had an inkling that my mother could lie to my face. For while it is Telstra's right to create a plan that the Federal Court considers to be poor value, it would be deceptive and misleading, bordering on illegal, if the company then pointed unsuspecting customers to that plan and told them it was the best one for their circumstances. Yet this was what Mum alleged.

The idea was absurd. The corporation that sponsors the Paralympics would not deceive a confused retiree by placing her on the worst-value plan in Australia. What would be its motivation? Money? I find it hard to believe that a company with Sol Trujillo in charge would put profit ahead of decency. Sure, according to the latest OECD figures Australia has the third-most expensive mobile-phone service in the world. But Telstra only has a 45% share of the market; it's not as if it exercises huge and anti-competitive power. I've seen the ads: Telstra's just in it for rustic farmers and bronzed surf lifesavers.

I was angry. In the Federal Court case, Optus had alleged that Telstra was pushing its less-informed customers to sign up to deals such as the $40 Phone Plan. I had dismissed these arguments as the rantings of a crazed foreign company - as Telstra points out on its "grass-roots campaign" website www.nowwearetalking.com.au, Optus is foreign-owned. And not just foreign-owned, but Asian-owned. By the Singaporean government. Which is Asian. And foreign.

It was time to expose my mother as a liar, once and for all. Together we walked into a Telstra shop and asked the nice man behind the counter which plan he thought Mum should be on. He looked at her, and then at me. He looked at her bill, and then at me. Without skipping a beat, he said that the Telstra $49 Cap Plan seemed most appropriate. Mum would pay no more than $49 per month. So much for her claim that she had been tricked. I began to suspect that she was foreign. Possibly Asian.

Later that day, as I angrily recounted this tale of motherly deception, my wife suggested that perhaps I was being harsh, that perhaps Mum's confused-yet-affluent demeanour made her seem easy prey during the original transaction, while my confident-yet-devilishly-good-looking demeanour had the opposite effect on the second occasion. And so I decided to give my mother the benefit of the doubt, and to see whether, under different conditions, Telstra would offer me the plan considered worse in value by the Federal Court of Australia.

I needed a double-blind experiment, one in which I couldn't see the Telstra employee and the employee couldn't see me. Also, the results of the experiment would ideally be recorded automatically, to save me the bother of transcribing the interaction later. Luckily, Telstra has devised the perfect tool for this: Live Chat online. I clicked on a button on the Telstra website and a window came up, and I was connected to Courtney.

"Hi, how may I help you with your enquiry today?" she typed.

"I don't really know much about phone plans but I want a mobile," I typed back. "Which plan should I get?"

After I outlined what I wanted - to make about 30 calls a month "to my children" - Courtney replied almost immediately with a long description of the $49 Cap Plan, including a series of legal disclaimers. She had apparently typed 146 words in fewer than ten seconds.

"Hang on," I wrote. "Are you human? Or is this a computer talking to me?"

For about 30 seconds there was no reply.

"Yes I am. My name is Courtney and I live in Townsville in QLD," she wrote. My belief in Telstra was renewed. Courtney had not suggested the plan that my mother had said she was offered, and was clearly just a very fast typist on occasion.

"OK, you pass the Turing test. Thanks for your help. I will get a $49 Cap Plan this afternoon."

"If you would like I can organise the plan for you?" Courtney replied.

"That's alright. I would like to do it with the nice man in the Telstra shop."

And with that we parted ways.

I remained curious, and wanted to find out about all the phone plans Telstra has available. In pamphlets I counted 18 different plans, but I had read online that there were others buried in the terms and conditions of some Telstra contracts. Unfortunately, the nice man at the Telstra shop didn't know, so I rang up the company's PR person, Peter Taylor.

The first time I'd called him, it was about Mum's $40 Phone Plan. He'd been brimming with confidence. "You're not going to believe the hype that Optus has put out on this," he had said, before reiterating all the standard arguments about how comparing the $40 Telstra plan with a really bad-value Optus plan would be a lot fairer. I'm not sure why Telstra feels this approach makes it look better, but Peter's manner was so reassuring that I nevertheless left the conversation feeling as though Telstra was a little Australian company being bullied by an evil foreign conglomerate, which, Peter reminded me, was Asian, as it was owned by the Singaporean government, which is Asian. And therefore foreign.

When I told Peter that this time I was ringing to find out the number of Telstra mobile-phone plans, his confidence evaporated: "You expect me to know that off the top of my head?" He had a lot of sub-answers: "We have hundreds of plans to suit everyone." But what about the plans hidden in some Telstra contracts? Like the $15 Talk Plan, which is not listed on Telstra's website but has a low call rate, standard text rates and only costs $15 in line rental? "I'll, er, have to get back to you on that one," he said eventually.

Being deceived is a terrible thing. It irks me that my mother would brazenly lie to me. It makes me re-evaluate all the things she has told me in the past. Perhaps it's not true that eating month-old chicken from the fridge will give me a stomach ache. Perhaps motorbikes are safe. Perhaps enlisting in the army would be the best thing to do.

The experience has upended my moral universe. But at least I still have one absolute. I know I can rely on Telstra.