In a comic panel, how do you show your readers that the voice is heard within? Do you use a different font, do you use italics, do you use bold characters? And how do you write this in a comic book script? I saw people use BOX: to refer to descriptions inside of box and then BOB: to refer to what people say, but I am not sure what you should do when the character or a character hears a voice from the inside.
As discussed in comments, you need to define this better as we're having trouble understanding if this is a narrative voice, internal thoughts, or off-panel dialog?
If this is a narrative voice, typically a "text box" or "thought box" is used. They are usually ascribed to a narrative voice of a character (in novel writing, think of this as a first-person perspective or a journal or letter written by a character). Less likely today, they can be used for a narrative voice, though this is no longer in style as a panel in a comic would be third-person objective style by its very nature, so a narrator would be a violation of "show don't tell".
Occasionally, the text box will be used for "Ed" to make comments (Ed isn't a person but short for "Editorial" and provides information to help the reader who might be confused). Typically the phrase starts with "Ed: [information]" with the asterisk linking it to dialog text. These are used to cite previous issues that are being referenced (i.e. Aracanid-Man: Professor Cephalopod?! I thought you died when you fell into that industrial deep frier" Text Box: Ed: See issue 198) or to denote that this dialog is being translated from another language that is not the work's primary language (i.e. Thug 1: <Tell him nothing. He doesn't speak Russian.> Aracanid-Man: English, MoFo, do you speak it?! Text Box: *Ed: Translated from Russian.) Text Boxes are sometimes also called "Yellow Boxes" as they tend to be yellow, often the same shade as post-it notes, though this is not always the case.
Superhero comics have a tendency to use a symbol of a superhero and do the box to mimic attributes of the hero's costume so their text boxes stand out from those of other characters (the Superman/Batman series does this to denote Superman and Batman's contrasting thoughts, with Superman's text boxes being more like a burst of sunlight while Batman's evoke a moon at twilight. The first box in an issue would also have their respective insignia fitted to the box).
Thoughts of the character that are fitting with the dialog are normally given thought bubbles (often a trail of puffy disconnected clouds leading to the "thinker" while the speech bubble is either round or similarly cloud-like). Contrast with speech balloons where the balloon is a defined oval with a cone like tail that leads back to the speaker. In setting, the speech balloon denotes spoken dialog whereas thought bubbles denote ideal dialog that for one reason or another is not said, i.e. you wouldn't mouth off like that to your boss who is throwing a fit. Some characters speak in nothing but thought bubbles, such as the titular cat in Garfield. His owner Jon does use speech balloons but because Garfield is a cat and cats don't talk, it's debatable if Jon is really understanding Garfield's retorts or is just reading (or failing to read) Garfield's very obvious retorts. Can be used to denote an implied communication by body language, which might be difficult to covey in a static drawn medium (think of how you know what a teenage girl is thinking when she rolls her eyes or what Ariel is trying to say during the period on the Little Mermaid where she is mute and cannot speak).
Dialog spoken off-panel but heard in-panel would use a speech balloon leading in the direction of the source of the dialog. If a speaker sneaks up on the visible character and says "What are you kids doing here?" the speech balloon's tail would lead to a portion of the panel that reflects general direction of the speaker in relation to the character (a speech balloon from a speaker who is behind the hero would usually be drawn to the lower part of the panel). If the character is eavesdropping, the balloon's tail would lead to the barrier that separates the hero from the speaker (a door, a window, the slats in the air duct's vent).
Telepathy or internal voices aren't your standard types of dialog so you're not going to find a definitive answer and you basically have to define your "style" yourself. However, there's examples of this in the past in text and visual media.
Sometimes alternative "quotation marks" are used to indicate telepathic or otherwise inaudible dialog. I've seen things like squiggly, square or angle brackets used. Although angle brackets (<>) have often been used historically in comics to indicate spoken language in a different language than the rest of the narrative so that might not be the best one to use, but you should get the idea.
- < hello >
In a visual medium like comics, a font that is different than the font used for all other dialog can be used. A differently designed dialog bubble with an outline style that is different than a spoken dialog or thought dialog bubble can be used. Color of the text or the background of the dialog bubble can also be used. You can use some internal dialogs early on between the character and the internal voice to firmly establish what is going on.
When writing the script, it may be beneficial to include a "Style Guide" that explicitly defines your needs. You might use it to define the label "INT:" or "TEL:" as indicating internal telepathic communication and then explicitly define what it should look like.
The important thing is to explicitly define your needs and what they look like and then stick to it.