Troubleshooting VoIP - connection lost, needs reset - ongoingly

My (limited) experience with VOIP is two-fold:  

1. VOIP systems with its own internal network wiring (not over the workstation wiring) and own Internet work effectively/
2. VOIP systems running over existing network wiring and sharing the internet with servers and workstations are subject to the problems above.

When you say that multiple resets daily are needed, what is being reset? Is it the phones, the router, the switch, or what?
How many VoIP connections are there? Are the computers piggy-backed onto the phones' network connection or are the LANs for the phones and the computers physically separate?

The customer has a Grandstream VoIP device, model DP750 - the User's Guide is at:
http://www.grandstream.com/sites/default/files/Resources/DP750_DP720_Administration_Guide.pdf

They get their VoIP service from voip.ms

The symptoms I saw reproduced this morning are:

- Sometimes a call just gets the voice mail when I can see that there is nobody on the line.  (There is only one phone in the office, so it's not a complicated setup.)
- Sometimes the caller gets a busy signal.
- Resetting the Grandstream device makes the problem go away.

I do need to contact Technical Support at voip.ms to see if they've seen this symptom.

Thanks very much.

Thank you for the update. Please let us know what voip.ms says.

Response from voip.ms:  There was an "easy fix", on the device's settings. The "register expiration" value was set too high.

The default's value is 1 hour and the recommended setting is 5 minutes. I just needed to log into the device's settings  and change that device, save the changes and reboot to apply.

Thanks, John and Perarduaadastra.