All calls go in 2 directions, inbound and outbound. Unlike the old-school copper-wire POTS calls, VoIP calls are connected through the internet. As such, when troubleshooting an IP phone that won't register or receive inbound calls, one must consider the local network, router/firewall, and internet connection.
With POTS, the connection is a dedicated circuit between the customer and the phone carrier. With internet calling, made possible by Session Initiation Protocol (SIP), the connection in between is shared. And, the dial tone on SIP phones is simply a user familiarity feature on the phones. While a dial tone does indicate a local network connection, it's not the same indication as a POTS line; just because there's a dial tone doesn't mean you can successfully make or received calls.
Successful calling on hosted VoIP service requires the call information to transit your LAN, your router/firewall, and the internet with as few issues as possible. If your phone alerts that it can't make a network connection, then there is likely an issue is on your local network.
A few tips:
SIP only works through 1 NAT; multiple routers will cause calls and registrations to fail. This is why in our OnSIP Knowledgebase, we review hardware that supports SIP communication.
The same goes for inbound calls: there has to be a clear path to the phones for calls to ring and connect. If there are any connectivity issues between our servers and your phone, they will affect call quality, possibly causing garbled words (due to dropped packets), sound delay, and distortion. If you experience any of these symptoms, you can check your packet loss by pinging us at sip.onsip.com. A 1% packet loss will cause audio problems.
Sometimes, internet connectivity issues are obvious. If you notice that your web pages are loading slowly or not at all and your call quality is that of 2 cans and a piece of string, then it's time to call your ISP and get them to fix the squirrel damage to the wires in your neighborhood possible. However, just because web pages are loading fine and the call quality is poor, doesn't mean it's not an internet connectivity issue. Check a speed check webpage like speedtest.org to see if you are getting the speed you pay for.
Another issue some experience with setting up a VoIP network is calls that go straight to voice mail without ringing. In our experience with OnSIP, this means we (the provider) can't get your phone to respond to our invites. To troubleshoot, make sure that your router allows UDP, which SIP uses.
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