Bittul Torah
Bittul Torah is when a person can learn, but doesn't, or when a person stops learning and gets caught up in trivial worldly matters, neglecting his learning. This is included in the posuk, "He disgraced the word of Hashem" (שו"ע סכ"ה ).
Bittul Torah applies to every single hour a person could've learned, but didn't (ח"ח, תורת הבית ).
Qualitative bittul Torah. Some say that a person who learns but doesn't put in the effort to delve into the Torah as much as he could is qualitatively mevateil Torah (מרן הגר"ח הלוי, משנת ר' אהרן ח"א עמ' נו ).
Disrupting Learning
Apart from the issur of bittul Torah, there is an issur to interrupt learning for something else. Doing so shows disregard for the Torah and disrespect for its kedushah. It is like speaking to a king and interrupting to speak to someone else (רבינו יונה אבות פ"ג מ"ז ).
The issur to interrupt learning is even for a single word, or a temporary interruption (תויו"ט אבות פ"ג מ"ז ). Therefore, one must be very careful not to use his phone while learning, not even to check who is calling, or similar, which would count as an interruption. It is praiseworthy to turn off the phone completely to avoid any temptation.
If one finished learning a topic and did not yet start another, he may interrupt if there is a need. Still, a ba'al nefesh should try to avoid even this, and wait until he is finished learning (יוסף אומץ הקדמון עמ' 268, שעה"צ סי' רפ"ה סקי"א ). After a seder, one may stop even if he is mid-topic (יוסף אומץ שם, ע' שו"ת שבט הקהתי ח"א סי' ש"ע ).
'Gezundheit'. One should not even respond "Gezundheit" to a sneeze in the beis medrash mid-learning (שו"ע סי"ז ). This applies nowadays as well (ט"ז סק"ו ושו"ע הרב פ"ד סי" א ).
Drinking. If one needs a drink to refresh himself, he may take a break, even mid-topic, to make a brocho and drink (שו"ת לב חיים ח"ג סי' כ"ג ). Similarly, if drinking a coffee or the like..., will help him learn better, he may stop for a coffee, as long as he actually feels he needs it.
Interrupting for Another Mitzvah
Chazal say that learning is equal to all the mitzvos (שו"ע סי' רמ"ו סי"ח). Nevertheless, sometimes one must stop learning to perform another mitzvah, as follows:
If the other mitzvah can be done by others, he should not interrupt his learning (שו"ע שם ). The same is true of a mitzvah only he can do, but he can wait until he is done with learning (ביאור הגר"א סי' ר "מ סקכ"ד ).
If the mitzvah can't be done by anyone else, he should pause to do the mitzvah, and then return to his learning (שו"ע שם ), e.g., burying a meis ל"ע , based on the niftar's honor (ע' שו"ע יו"ד סי' שס"א ס"א ); chesed; tzedoko; and hachnosas orchim (אהבת חסד ח"ג פ"א ); or gladdening a chosson and kallah (רמ"א אבהע"ז סי' ס"ה ס"א ).
