Hi there and welcome
Can you please unplug the phone from power and the telephone line and let it sit for 10 minutes.
Then plug the power and line back in and tell me if the speaker function now works please.
It is the internal control board or speaker that has faulted.
A corrupted firmware can cause this. But we have ruled this out by trying to hard restart it.
I am really sorry about this and you are going to have to return the unit to have it replaced. It will be covered by the warranty if this has not expired, and the replacement will be done at no cost to you.
I am really sorry about this, and if you have problems, or would like any additional information or assistance, please do not hesitate to let me know.
And if you can take a second to rate my service, that would be greatly appreciated.