It sounds like your router is blocking / not replying to SIP re-invites at certain time intervals.....

Read this similar issue from a customer at Phonepower:

http://www.phonepower.com/forum/topic.php?id=1190

The tech says "The most common reason for calls dropping around the 10 - 15 minute mark is due to the device not receiving / or replying to the re-invite our server sends out to keep the call leg active. The way to resolve this is to either configure the nat so we have direct access and no ports being blocked, or to bypass any router and connect the ata directly to a cable modem that does not have any layers of nat."

You'll probably find this issue goes away if you directly connect the ATA to your cable/DSL modem (bypassing the router). This is not an option in your case since you won't be able to connect your router anymore.

Try these steps in your router:

1) Disable SIP ALG. Log into your router and look under the different tabs/settings and see if there is an option to disable "SIP ALG". This option is usually found under the "Administration" tab on Cisco routers, but not always. If you can't find the option, then it's likely builit-in to the router's firmware and can't be turned off. If this is the case, you may want to consider changing the firmware to DD-WRT which has the ability to turn this feature off.

2) Log into your router and assign your ATA a static IP, then either forward UDP ports 5000-65000 to the ATA's assigned static IP or place it in the router's DMZ. This may resolve your issue, but keep in mind this exposes the ATA with little to no firewall which can cause unwanted SIP probes/scans from hackers.

Since you are BYOD, I'm not sure how much Voipo support team can help but it doesn't hurt to ask....