Why would anyone be that daft to use their own phone? With how people use the dark Web and other things surely it's not that difficult for him to communicate with United his recommendations. In fact regular meet ups with Brailsford, his long time friend, can be defended as they knew each other before all this - what stipulation is there in his contracts that limits him from physically meeting his old friend to catch up every now and then?
They wouldn't, but that's not really the point. I've acted in cases like this, and if you're interested, the process generally works like this.
The value in these clauses for high level executives/key people in a business is the threat that an employer with deep pockets and who wants to make a point,
will pursue an injunction/damages if they suspect you are in breach. I could have an application drafted, in to Court and heard the next day to seek an interim injunction. Then the process kicks in and everyone needs to prepare for a return hearing where all the evidence is aired. If there's any substance to the allegations it'll come out then because (in my experience) it always does.
The risk to anyone subject to this (putting aside the cost of your own lawyers, and risk of having to pay a fortune in costs/damages at the end) is that they have to stand up in Court and have two options: tell the truth and see if a Judge thinks your actions put you in breach based on the facts, or don't tell the truth. If you're caught out on the latter you're potentially going to prison, you'll certainly lose the job you had lined up, and probably struggle to get another one.
The club won't encourage him to do anything that might lead to the above, because it would also be party to any proceedings and someone there would have to give evidence and the same risks apply.
To answer your question, there is absolutely nothing stopping one person from meeting another. The advice any lawyer will give to either party would be not to do it, because whether it is innocent or not, it'll be perceived to be the latter. You only need to convince a Judge on the balance of probabilities in cases such as this,
i.e. 51% or "more likely than not" and you'd probably say that's realistic in the circumstances.
These clauses do work in practice where employers aggressively police it, mainly because people are not willing to risk their own money and reputation to benefit their future employer. The fear is that they turn on you if the previous employer takes action.