Check the following:
1. If its on one extension, the extension is likely to be set on DND (Do Not Disturb).
Some phones have a designated key for this function. If you don’t, you need to dial *79 to stop this service and *78 to activate it again.
2. If its a general issue in the office, and after changes done by your ISP, make sure they have VOIP ports open.
You also call us and we can help you to contact your ISP.