You may want to consider a few different things before making changes to the VoIP system or touching the configuration on phones.
Confirm that there are no issues with the speed of the customer/end-user/s Internet connection. Quite a lot of VoIP issues are caused by high latency or packet loss on the Internet connection
Is there any devices that the phone connects through to get to the internet other than the modem/router itself? (i.e. any switches) Check to see if the issue lies with one of these devices.
Below is a list of some common issues & problems that you may encounter, as well as a few steps you can try to do in order to resolve them before calling support.
Things to test/check when encountering "Crackling / Static on phone call/s"
Run an Isolation test if at all possible (Only have one of the phones connected and try to replicate the issue)
This is primarily to check if the issue occurs only when there are more than one SIP device within the network, or if the issue is to do with a specific phone
Change the phone from using "UDP" to using "TCP"
You can find these settings by logging into the phones "WebGUI" and navigating to "Accounts > Account X > SIP Settings"
The reason for this is that the TCP protocol allows the packets to be tracked end-to-end which can reduce packet loss but tends to require a little more bandwidth
Change the VoIP codec to "G729a" temporarily to see if the crackling/static issue improves
The G729a codec uses higher compression and less bandwidth
NOTE: this codec will cause a decrease in the audio quality of phone calls and as such should not be used as a permanent solution
This is usually caused by NAT issues (The device getting in/out of the network to the hosted PBX)
Check that "SIP ALG" is disabled on the router
If "SIP ALG" is enabled you will need to disable it then reboot the router, once the router comes back online you will then need to reboot all of the phones.
Rebooting the Router and Phones is important after changing the ALG setting as the devices will retain the old settings and NAT tables until they are rebooted.
NOTE: Some routers don’t allow you to disable SIP ALG, in this case it's recommended you replace the router with one where you can.
Set the local SIP port to be something other than "5060"
You can find this setting within the "WebGUI" of the phone under "Account > Account X > SIP Settings > Local SIP Port"
Sometimes this helps with the "UPnP" settings in routers as some don’t like having multiple devices using the same "UPnP" port
This issue is usually caused by the UPnP ports with the modem/router closing at an interval that is lower than what the phone is set to renew it at.
Change the registration interval from 60 minutes down to between 1-5 minutes
This setting is located in the phone by going to "Account > Account X > SIP Settings > Basic Settings", the field that needs to be changed is “Register Expiration”
Some routers will remove the "UPnP" port quicker than 60 minutes which can cause the phones to be unable to communicate with the VoIP server but still think they are registered.
Setting the registration interval to a lower setting makes the phone re-register with the server more often which keeps the "UPnP" port alive
There are multiple things that may cause SIP registration to fail, including being unable to access the VoIP server at all from the internet connection (i.e. routing issues)
Confirm that you can ping and browse to sip01.mhn.net.au
This will confirm whether there is a routing issue on the connection or not
Reset the SIP Password and re-enter it into the phone.
This will eliminate any possibility of an incorrect password.
Reboot the phone or PBX (if it is a SIP Trunk) to force re-registration