I have 2 users on a Summit 800 who state they are "all of a sudden" unable to dial out. They phones sets are IP. I have checked the class of service but it looks the same as all the other extensions.
Not sure where to go with it from here.
Any guidance would be appreciated.

Are their mute buttons flashing by any chance?

Did you verify that their button programming is correct?

I am having a tough time contacting the 2 users. The buttons are the same as on all the other phones. 3 instances of U-Loop. Cass of service is the same as other extensions. Day, Night and Timed is 2.

On rare occasions, I have had phones inadvertently placed in Temporary COS. If this is the case, the Mute button will be flashing.

Interesting. I will put my hands of the phones tomorrow. Then I will try to make calls to see where they're going.
TYVM for input

If it is Temporary COS, here's how you unlock it:

To unlock the phone: [TRANS/PGM] 22 + Password. (Password = ext + v.mail password, example 1010000)

Can an analog station do this also?

evidently it needed a clip ID.