You can force caller ID however you want. It is just a matter of the specific customization.
By default, these are your only options. You would have to get into a custom Asterisk context to do more.
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And your iPhone will show is as a number from what ever country code matches the first part of the extension. (5100 will show as Peru).
I used a CID prefix in one situation. doesn't come through to my cell phone, but passes through an IAX2 trunk to my desk phone nicely.
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