Having this information in the back of your mind as your configuring your phones can save you hours of time troubleshooting later.
Definitely make this list one of the first things you review in your troubleshooting process.

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Use DNS NAPTR and SRV records.

These records were designed to make getting your device connected as easy as possible.

It automatically gives the phone many of the most common best settings.

1. Which transport is available: UDP, TCP, TLS

2. Servers to connect to with load balancing and failover priorities

3. The port to connect through, port 7000 instead of SIP default 5060

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FASTDEVICE Outbound Proxy Servers.​

Select the one for your region. 

​1. us-east.fdvoip.com, port 7000

2. us-central.fdvoip.com,  port 7000

3. us-west.fdvoip.com, port 7000

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FASTDEVICE SIP Server Host.​

1. <your realm>.sip.fdvoip.com, port 7000

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Connection setup on the phone

1. Check if your device supports NAPTR DNS records. Enable/Use them. This often requires (device-specific) setting the device to utilize NAPTR AND requires you to set the port to 0. Check the device manual.

2. If the device does not support NAPTR, check if it supports DNS SRV records. Enable/Use them.

3. If the device does not support NAPTR or SRV, manually use port 7000 for SIP when connecting to us. Anywhere you see a setting which connects to port 5060 on our servers (both proxy and realm), change it to 7000 (ONLY if not using DNS NAPTR or SRV records). 

4. Change the LOCAL SIP PORT of any device you're setting up to be above 10000 and an even number. Some applications don't expose this setting. 

5. ALWAYS try UDP first. If UDP works, use it! This is important because it allows our own systems to undergo maintenance, failures, network connectivity issues, etc. without kicking the phone offline. TCP is also very expensive on system resources. HOWEVER, if UDP simply won't work, or if the device will be traveling a lot to locations with strict firewall rules, you can change the settings to TCP.

6. Make sure the device is registering every 300 seconds, but NO MORE than every 300 seconds.

7. If the device has the ability to send keep-alives, turn that on.

8. If the device has  STUN  / TURN / ICE options, turn those OFF. This is contrary to some articles on the  internet  but these features are usually a detriment and can degrade call quality or cause slow call setup times. We already do most of this work on the server side, no need to do it again on the client/device side.

Firewall/Router and Modem Configuration

1. If you have control of the modem and/or firewall/router, ensure SIP ALG is turned OFF. Note that both the modem and the firewall/router may have this feature and both must be off.

2. Many routers, modems, and service providers have built in applications to supposedly assist with VOIP calls or sometimes even block VOIP calls. They almost always just cause problems. Using SIP port 7000 to connect to our servers and a high LOCAL SIP port usually avoid those problems. They should be checked for and disabled if there are any problems.

3. If you are behind a Sonicwall firewall, ensure "consistent NAT" is enabled.

Audio settings on the phone

1. If you have audio issues, try disabling HD audio codecs. Use PCMU.

2. If you expect the device to be used in low-bandwidth environments or on roaming WiFi, use G729. There is a slight quality decrease if you do this. Remove PCMU altogether if you do this to force G729.

3. If you expect the device to be used in low-bandwidth environments or on roaming WiFi, change the Jitter Buffer to Fixed and set it to the highest level possible (depends on the app as to how this is displayed and implemented). This allows packets to arrive later, at the expense of audio being more delayed.

4. If the device has an RPort setting, turn it on.

5. If the device has STUN / TURN / ICE options, turn those OFF. Same as for the SIP signaling.

6. If you have control of the modem and/or firewall/router, ensure SIP ALG is turned OFF. Note that both the modem and the firewall/router may have this feature and both must be off.

7. Check Internet connection for congestion and quality.