Many older or inexpensive home and small office routers don't handle VoIP phone traffic properly.  This does not apply to our managed services or most enterprise-class routers.  The symptoms may be some or all of these:

Can receive calls for a few minutes after rebooting the phone or router, then inbound calls go directly to voicemail.

Phone won't work at all in that location, but a test phone in another location works.

Can dial out, but not receive calls.

One person can hear the other party, but the other party cannot hear anything.

There is no ringing when dialing, then suddenly the other person is on the call.

Here are a few suggestions:

1.  First, simply reboot the router, and see if the problem comes back.  Sometimes it may not.  Or it may be a problem that returns weeks or months later.

2.  Many routers have a "VoIP" section with setting just for phones.  In there, you will usually find either one or two settings.  They may have varying names, but will sound similar to "enable VoIP helper" and "enable consistent NAT."  Other common terms are:  Enable NAT transformations, enable SIP ALG, enable SIP helper, etc.  You will want to try different combinations of settings to see which one fixes this issue.  Most routers will have a set of settings that fix the problem, but occasionally you may find an older router that does not.

3.  Update the router's firmware.  If the software version is more than a couple years old, do an upgrade according to your router manufacturer's instructions.  Many times, fixes and new features are deployed in new versions.

4.  You may need to replace the router.  Some simply have defective NAT software that doesn't properly pass VoIP traffic.  Some Actiontec routers from CenturyLink simply don't work properly, and they will replace it at no charge.

As always, please feel free to contact us if this didn't answer your questions.  While we simply don't have familiarity will all routers out there, we'll do what we can to help further.

Here are routers KNOWN to solve this problem:

Apple Airport Extreme (any model, with current/recent firmware)
All ASUS, Netgear, and Linksys 802.11ac routers
Sonicwall (any current or recent model)
Mikrotik (any model from about 2010, with current firmware)

Routers KNOWN to cause problems:

Older Sonicwall routers (about 2012 or earlier), or with incorrect settings

D-Link DIR615

Older Cisco/Linksys routers supplied by the cable/phone company

CenturyLink/Qwest combo DSL modem/routers (Actiontec primarily)

Older Cox combo modem/routers (older than about 2011)