In the context: I told him about I bought a new mobile on the phone.
A: Unfortunately, I missed your call. May I ask what the purpose of your call was?
B: I just had to let you know about I bought a new mobile so that I called you.
The combination "... " is not possible. "about" (in this sense) needs to be followed by a noun or noun phrase.
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The combination "... about I bought ..." is not possible. "about" (in this sense) needs to be followed by a noun or noun phrase. Thus, you could say:
I just had to let you know about the new mobile that I've bought
Alternatively, you could say:
I just had to let you know that I've bought a new mobile
At the end of the sentence, say "..., so I called you" (