There are a few things to check first:

- does it work with other phones or is it just this phone that does not work?

- does it work on other networks?

- are you using a 790X/791X with hands free? these phones only have monitor, not speaker phone

- if you are dialing with the handset onhook then lifting the handset, make sure your hookswitch is disengaging properly

- if you have ACLs, be sure to have a "log" on any deny statements so you can be sure it's not your ACLs

- check Device license