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There are three Htek IP Phones: A, B and C

1. Use Call Park feature via 3CX

(1) Login 3CX system → Setting → Call Parking → Get "the Call park place number"
e.g.: SP10
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(2) Login IP Phone A's webpage → Function Keys → Line Key/ Memory Key → Set Type to "Call park", fill "SP10" in the Value field
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Login IP Phone B's webpage → Function Keys → Line Key/ Memory Key → Set Type to "Call park", fill "SP10" in the Value field
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(3) C calls A, A answers, then A presses the Call park key, C will be held.
(4) B presses the Call Park key, then B will talk with C.

2. Use Call Park feature via MyPBX

(1) Login MyPBX system → PBX → Basic Settings → Feature Codes → Call Parking Preferences → Get "the Call park feature code"
e.g.: *6
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(2) Login IP Phone A's webpage → Function Keys → Line Key/ Memory Key → Set Type to "DTMF", fill "*6" in the Value field
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(3) B calls A, A answers. A presses the DTMF key, then A will hear a voice prompt about parking position (The parking position is usually "690") and hang up. B will be held.
(4) C calls "690", then C will talk with B.
PS:
Call park LED: