Thanks,
I have had some users experience similar issues at times. Usually, it had something to do with them using the speakerphone function, they were dialing the wrong password, or they dialled the password too soon or quickly.
Do your users eventually get in? If they eventually get in, then it is most likely not the password being incorrect.
To test, I would first try with the handset. The speakerphones on those sets are not the best and if memory servers correct, it is only half duplex. That means that you only get one way audio depending on which end is transmitting. If it is good there, then try again on speakerphone, this time turn off the microphone. The microphone may be picking up something that is interupting the call.
Good luck!
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My Biggest problem is that I almost always believe what I tell myself.