There are at least two good reasons why cell phones are not allowed in tournaments.
One is that, with the readily available chess programs/engines and texting availability on almost all cell phones, text messages can be sent with computer engineered moves either from the computer or from a co-conspirator (such as “play 10.Rae1, idiot!).
Back around 1990 I was participating in an OTB tournament and had a cassette player with earphones and listening to some inspiring music. I easily won the game.
But immediately after the game my opponent strolled over to the tournament director (TD) and told him that he suspected I was cheating. He complained that I could be listening to pre-recoded moves coming from my cassette player. I looked at my former opponent, and gave him a look that very much suggested, “you’ve got to be kidding”.
In the presence of both the ex-opponent and the TD, I took out the cassette and showed it to the TD. The TD was satisfied about the label on the cassette and was about to rule in favor. But my ever suspicious opponent claimed I could have erased the content of the tape and replaced with my voice saying, “1.e4 e5 2.Nf3 ..”.
I put back the cassette in the player and showed the TD how to play it. He did so and briefly listened to The Grand Illusion album by Styx. The TD smiled and then handed back the player back to me. And while I walked away, cleared of any wrongdoing, I noticed the TD slowly shaking his head.
Now let’s go to the second reason why cell phones are not allowed in tournament halls. The game was played on-line as there is a nasty virus going around.
chess.com, Sept. 1 2020
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.c3 Nf6 5.d4 exd4 6.cxd4 Bb4+ 7.Nc3 Nxe4 8.O-O Bxc3 9.d5
(This is Moller Attack. The main line goes 9…Bf6 10.Re1 Ne7 11.Rxe4 d6, reaching a well-known position. And too drawish in my opinion. I therefore played an offbeat and interesting move and found out after the game there is theory on it.) 9…Na5!? 10.bxc3 Nxc4 11.Re1
(More common is 11.Bd3 or 11.Qd4.
Nuremberg Open, 1990
11.Qd4 Ncd6 12.Ng5 Nxg5 13.Qxg7 Nge4 14.Qxh8+ Ke7 15.Qe5+ 1-0
Nova Gorica, 2001
11.Bd3 Nf6 12.Bg5 h6 13.Bh4 d6 14.Qa4 b6 15.Rfe1 a6 16.Re3 Bd7 17.Qc2 Nb7 18.Rae1 Nc5 19.Bf5 Ba4 20.Qd2 g6 21.Re7 Kg7 22.Bxf6+ Kh7 23.Ng5+ Kg8 24.Ne6 Qxe7 25.Bxe7 fxe6 26.Bxe6+ Nxe6 27.dxe6 Kh7 28.Bxf8 Rxf8 29.e7 Re8 30.Qf4 Bd7 31.Qf7+ Kh8 32.Qf8+ Kh7 33.Qf7+ Kh8 34.Qxg6 h5 35.f4 1-0
Even the text move has a precedent.
Pierre Francois Geronimi-GM Loek Van Wely
European Blitz Ch.
Ajaccio, Oct. 25 2007
11.Re1 Nd6 12.Ng5 O-O 13.Nxe4 Nxe4 14.Rxe4 d6 15.Qf3 Re8 16.Rxe8+ Qxe8 17.Be3 f6 18.Re1 Bd7 19.Qg3 Qe5 20.Qxe5 fxe5 21.c4 b6 22.Bg5 h6 23.Bh4 g5 24.Bg3 Bf5 25.Re2 Bd3 26.Rd2 Bxc4 0-1.)
11…Nd6! [A more-or-less thematic move in this variation. It (temporarily) puts a stop to White’s plans and Black doesn’t mind giving back some material, as long as he stays ahead. Side note: It’s nice while checking the game against a database to find out that a move that you found OTB is identical to one that a GM played. But let’s get back the game – I have yet to win the game.] 12.Qc2 O-O 13.Rxe4 Nxe4 14.Qxe4 Re8 (White has a lead in development for some material. But he wastes tempi in his next few moves by trying for a quick mate.) 15.Qg4?! d6 16.Qg3 Qf6 17.Bb2 Bd7 18.Rb1 (Attempting to gain the momentum after 19.c4. But Black is ahead of him.) 18…Qg6 19.Qxg6 hxg6 20.c4 Re2 21.Kf1 Rc2 22.c5 Rxc5 23.Rc1 Rxc1+ 24.Bxc1 Re8 25.Be3 Bb5+ 26.Ke1 (I’m contemplating about White’s possibility of taking the a6-pawn. Oh wait! Is the bishop pinned? I don’t think I need to worry about my a-pawn just yet. Is there a good move for Black here?)
(Ring!! The cell phone goes off!! Do I need to check the phone? And I know I must make a move as this is a speed game. I quickly figure that any move that doesn’t immediately lose should be OK.) 26…a6? (And I fell back into thinking my a-pawn is under attack. Black has the much better 26…Bc4! 27.Kd2 Bxd5 28.Bxa7? b6, which might let me finish the game and answer the call at the same time. But I missed this golden opportunity. So I decided to say, “hello”, keep my eye on the game, and tell the caller I will call her back in a few minutes – but not tell her I’m playing blitz chess.) 27.Kd2 Bf1 28.g3 Bg2 29.Nd4 (Back on track and everything going good so far. Then my caller asks me a question.)
29…Bh3? (Eek! Loss of concentration and loss of a free pawn and a tempo. Black has the better and obvious 29.Bxd5! I tell myself that at least I took some squares away from the knight, but I know that’s not the reason or issue. I had let my concentration drift again.) 30.Nc2 Bg2 31.Nb4 a5 32.Nc2 Bxd5 (Finally! Now White is lost. And I realize I have more time. Maybe Black also has a phone call??) 33.a4 Bc6 0-1