Learn Hardcore Chinese: tā méi kànjiàn wǒ, yīnwèi tā yìzhí zài dǎ diànhuà. - He didn't see me because he was on the phone the whole time.

Use 没(有) to negate completed events, existence, or achieved results. 看见 is a resultative verb (“see/notice successfully”), so to say the result did not occur, you use 没(有): 他没(有)看见我. 不 is for habitual, general, or volitional negation (e.g., 他不看手机 “he doesn’t look at his phone”).

Should there be 了 after 看见? Why not 看见了?

In the negative you cannot use perfective 了: 他没看见我 is correct, but 他没看见了我 is wrong. In the affirmative, either is fine: 他看见我了 or 他看见了我. Both mark the event as realized; word order slightly shifts focus, but both are common.

What’s the difference between 看见, 看到, and 看?

看见 (kànjiàn) and 看到 (kàndào) are both resultative: “manage to see; end up seeing.” In most everyday contexts they’re interchangeable.
看 (kàn) alone is “to look/read/watch,” not necessarily implying a successful “seeing.” Here you want the achieved result, so 看见/看到 is better.

How is 没看见 different from 看不见?

没看见 = a factual non-occurrence: “didn’t see (on that occasion).” Often accidental or due to inattention.
看不见 = inability: “cannot see” (too dark, blocked, bad eyesight).
Given “because he was on the phone,” 没看见 is the natural choice.

What does 一直 add when I already have 在? Are they redundant?

They stack to emphasize continuity:

在打电话 = in the middle of calling (progressive, at that time).
一直 = continuously/throughout a stretch of time.
Together, 一直在打电话 = “was on the phone the whole time.” You can also say 一直打电话 (also fine), or just 在打电话 (less emphasis on duration).

Can I use 正在 or 着 instead of 在?

正在 is a bit more explicit/neutral-formal than 在 for “right now”: 他正在打电话.
着 marks a continuing state and often sets background: 他打着电话没看见我 (“He, being on the phone, didn’t see me”). 打着电话 is less common than 在/正在打电话 for simple progressives.

Where can the 因为 clause go? Do I need 所以?

Both orders work:

Cause first: 因为他一直在打电话，所以他没看见我。
Result first: 他没看见我，因为他一直在打电话。
When the cause comes second (as in your sentence), you usually don’t add 所以. When the cause comes first, adding 所以 is common but optional.

What does 打电话 literally mean, and how do I say “call me” or “answer the phone”?

打电话 literally “make a phone call; be on the phone.”
“Call me”: 给我打电话 (Mainland), or 打给我 (common in Taiwan).
“Answer the phone”: 接电话.
Also useful: 挂电话 (hang up), 回电话 (return a call).

Are both 他 in the sentence the same person? Could that be ambiguous?

By default, yes—the two 他 refer to the same “he.” Context usually disambiguates. If needed, replace with names or nouns: 小王没看见我，因为小王一直在打电话。 Or change one pronoun if the referents differ.

How do I say “He hasn’t seen me yet” or “He has never seen me”?

“Hasn’t seen me yet”: 他还没(有)看见我。
“Has never seen me (ever)”: 他从来没(有)看见过我。
Note 还 (“yet”) and 过 (experiential) change the meaning.

Why isn’t 过 used here?

过 marks life experience (“have ever done”). Your sentence describes a specific occasion, not life experience, so 过 isn’t appropriate. Use 过 for things like 他没见过我 (“He has never seen me before”).

Can I drop subjects or objects in this sentence?

Yes, if context is clear. Examples:

没看见你，因为他一直在打电话。 (subject dropped)
他没看见，因为一直在打电话。 (object dropped) Chinese frequently omits recoverable elements.

Is the comma before 因为 correct? Could I make it two sentences?

Yes, a comma is natural between the main clause and the reason clause. You could also write two sentences: 他没看见我。他一直在打电话。 The single-sentence version with 因为 sounds more connected and natural.

Any pronunciation tips for the whole sentence?

Tā méi kànjiàn wǒ, yīnwèi tā yìzhí zài dǎ diànhuà.
Notes:

看见 is kàn-jiàn (both 4th tone).
一直 is yì-zhí (tone sandhi: 一 becomes yì before a 2nd tone).
电话 is diàn-huà (both 4th tone).

Can I say 他一直电话?

No. 电话 is a noun; you need the verb 打 with it: 他一直在打电话 or 他一直打电话.

Would 没注意到 work instead of 没看见?

Yes, 他没注意到我 means “He didn’t notice me,” which highlights attention rather than visual perception. With the phone as the reason, 没注意到 is also very natural. Use 没看见 for literal seeing; 没注意到 for noticing.