Today the Talmud tells us that an individual should not hold their Tefillin (phylacteries) in their hand or even a Torah scroll in their arm while praying. This is talking about the Amidah, the silent prayer. One should not hold these items, whether Tefillin or a Sefer Torah (a Torah scroll), in their hand and pray. Shmuel comes along and adds a line later: a knife should not be held, money should not be held, any type of vessel such as a platter should not be held, and even bread should not be held in one's hands while praying.
So Rashi explains on the page that the reason for this is that a person will not be able to concentrate properly in their prayer when they are holding any of these items. Why? Because they are going to be scared that it might fall. Obviously, a person holding a Sefer Torah or Tefillin, or any of these items, would not want them to fall. So they are preoccupying part of their concentration, part of their brain, with holding, guarding, and keeping safe whatever they are holding in their hand. Again, a knife, obviously, money, a platter, a plate, even bread: a person does not want to lose any of these things or allow them to fall, lest they break or get lost.
So imagine speaking to someone while you are texting on your phone. Oh, well. Sorry. That happens all the time, unfortunately, due to the lack of respect that we have nowadays. Okay, so what about someone you respect, someone you revere, someone you owe it to, someone maybe very special or famous whom you may be meeting? You are not going to speak to them and text at the same time. You are going to put your phone down, look them in the eye, and speak like a person. Well, Hashem is no different. At least, He has to be treated like them. When we speak to Hashem, when we pray to Him, when we praise Him, and when we request different things from Him, the minimum we can do is focus on what we are saying. The bare minimum we can do to focus is during the silent prayer, during the Amidah, the most important part of our prayer.
But since prayer is so powerful, the Yetzer Hara, the evil inclination, does all He can do to have us lose focus and think about anything and everything except the words we are saying when we are praying. All of a sudden, people have amazing revelations and ideas and great prophecies, even in the middle of their Amidah. Why? It is not by chance, and do not worry, you are not the only person. It is because the Yetzer Hara attacks at the most important times and puts all of his efforts into the most important times. The silent Amidah. The silent Amidah. The silent Amidah is very, very important, the most important part of our prayer, and that is when we need to fight at our strongest.
The Talmud is teaching us that we have an obligation to do everything to set ourselves up for success, to concentrate during prayer. If it means choosing your seat wisely, choosing a proper place to pray, not having any distractions present, or at least avoiding them, holding something—the Talmud refers to is a cause for distraction. So now, according to Halakha (Jewish law), anything that would add to one's focus is permitted to be held during the silent prayer. For example, a Siddur, a prayer book. If a person is going to hold a prayer book and read from it to concentrate and connect more to the words and to what they are saying, then they are obviously allowed to hold it.
Holding a cell phone in the middle of one's prayer is completely prohibited. It should either be put in one's pocket or on the table near where they are praying. Whether it should even be brought into the synagogue at all is not what we are speaking about now. But if a person is using their cell phone as a Siddur, meaning there is some type of Siddur app on it, then it would definitely be permitted to use it, because they are using the item at hand—the cell phone—to help with concentration in prayer.
There is one more example where you are allowed to pick something up, and that is the law that you are not even allowed to hold your child in the middle of your Amidah, or start off your silent prayer holding your child, with the exception of a crying child. If the child will cause you and those around you more distraction—if they are running up and down the aisles, crying, and ranting—then it would actually add to your concentration and the concentration of those around you if you pick the child up. You are not only permitted to pick up the child; you are obligated to pick up the child.
These are two or three examples of when you are allowed to hold something. Although the Talmud says not to hold anything because it will distract you from concentration, if holding that item will add to your concentration, you are allowed, and it is preferable, to hold it.