IP电话机报错“Extension In Use”

公司IT给装了一个IP电话，但是很快就发现就不能用了，拿起话机就嘟嘟只响。显示Extension In Use。。。死活也不明白是啥意思。goolge了一下，看到AT&T公司有这种提示：

FAQs

Problem

Display shows "Extension In Use".

Solution(s)

Please review the below solution(s) to assist with the problem(s) you may be encountering.

1. Line voltage is incorrect. Shows "Extension in Use".
   
   The line voltage on the customers line is too low. "Extension in Use" will appear on the screen when the line voltage drops below 21 volts. Normally, this would only happen when another phone on the line was off hook. Contact your local telephone company for assistance; additional charges to be inclurred by customer may apply.
   
   
2. Normal Operation. Display shows "Extension in Use".
   
   Normal Operation. The unit will show "Extension In Use" when another set on the line is in use. It shows this because the unit has sensed a change in line voltage. If there is not another phone off hook when it shows this message, that is due to a temporary fluctuation in the line voltage. There is nothing malfunctioning with the unit.

大意应该是与别人冲突，或者是电压不够。后来IT来修理的时候也确实发现这一点了。就是我的一个同事也注册了这个号码，导致冲突了。后来，给他重新换了一个号码，问题解决。