Why they write the word 'Call' as /kɑːl/ (Cambridge)
And some others, they write it like /kɔl/ (Collins)
So, are they the same in American English?
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Let us quote your sources in a little more detail:
Cambridge: "UK /kɔːl/ US /kɑːl/"
Collins: "in British English (kɔːl) [...] in American English (kɔl)"
This appears to be a case of allophone variation, and the use of IPA to spell pronunciations phonemically and not phonetically.
In Cambridge, the sound in "call" is spelled /ɑː/ In Collins the same phoneme is spelled /ɔ/
Allophonic variation is different pronunciations of the same morpheme that native speakers are not completely aware of. For example "r" and "l" are distinct in English, but allophones in Japanese. /ɑː/ and /ɔ/ are distinct in British English, but not in American. When designing a phonetic alphabet for a language, the dictionary writer has to choose one symbol to represent a particular phoneme, even if it is produced differently by different speakers. This is the difference between a phonemic transcription and a phonetic transcription.
So while [ɑː] and [ɔ] are different phonetically, they can represent the same phoneme in American English.
You can compare the sounds in ball and box (in American English). Listen to them. Does the vowel sound differ only in length or does it have a different quality. Also listen to ball (BrE) and ball (AmE) does this have a different sound to you, or does it differ only on length?