1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method (7.X.X?) and vicidial version with build (VERSION: 2.X-XXXx ... BUILD: #####-####).
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "manual/from scratch" you must post your operating system with version (and the .iso version from which you installed your original operating system) plus a link to the installation instructions you used. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) Ringing requires valid, active, registration. Registration expires/renews at timed intervals based on settings in Asterisk and in the VOIP phone. But the firewall between the agent and the server will require activity on the "registration port" to keep that port open. If there is no activity for too long, the port will close. If the firewall/router setting for "how long do I keep rports open" is lower than the "registration time": The port will close before the re-registration occurs. Asterisk, however, will be blissfully unaware of this and send the call to that closed port. The router will discard the ring packet ... and the VOIP phone will not ring.
Test the theory: Have any agent whose phone does not ring UNregister and REregister (NOT just close the phone window and reopen it, UNregister!), then log in again. If that ALWAYS fixes the ring problem, then this is your issue.
Solution(s): Modify the VOIP phone registration timeout to a lower number (In Zoiper's settings) so re-registration occurs before the router's port timeout closes the port OR increase the amount of time the router keeps those rports open. Often the VOIP phone's registration timeout can be done in the Asterisk configuration file instead of the soft phone.
In all cases, however, this is an Asterisk question more than it is a Vicidial question (much larger community there). Also note that Zoiper speaks both SIP and IAX2, and it makes a difference which one is in use as the settings are different. Zoiper also has many versions and there are lots of videos/instruction sets online to alter settings.
Happy Hunting