Regarding the IP phones and analog phones in the U1900 system, which of the following statements is correct ()

Regarding the IP phones and analog phones in the U1900 system, which of the following statements is correct ()
A . If you want to successfully register IP phones A and B to the U1900 system, the U1900 system must include the MTU board
B . When IP phones A and B have successfully registered to the same U1900 system, if IP phones A and B are able to dial each other, the U1900 system must include an MTU board
C . When IP phone A and B have successfully registered to the same U1900 system, if IP phone A calls When calling IP phone B, IP phone B is busy, then if you need IP phone A to hear busy Line tone, the U1900 must contain MTU board
D . If IP phone A and analog phone C (directly connected to the ASI board of U1900) are successfully registered to U1900, when A calls C, U1900 may not contain MTU board

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Answer: C