If your agent is not able to receive calls, first check that they have their phone number configured properly in My Profile on the agent desktop. The agent's status must be in work offline to make changes to their settings. Agents using DID, PSTN, cell, or other landline phones must always have a '1' in front of their number in their profile. For example, 12223334444. If using an Eyebeam phone, change My Profile to use a cell phone number or other working landline phone and try a verification call and/or an outbound call.
Try making a direct outbound phone call from the agent's phone outside of the application.
Using a cell phone or other working landline phone, try calling the agent's phone number they are using with the application. If the call does not complete, then you will need to contact your local phone provider. If you get the agent's voicemail, verify that the agent does not have the phone set to forward calls to voicemail or that the agent's vm and prompting timeout setting in the CM is set for a lower setting than the PBX voice mail setting. If this happens and you do not know how to clear the forward, you will need to contact your PBX/local provider.