Please find below what needs to be completed on your Company/Home modem or router/firewall. This can be accomplish by your IT department or by contacting your Internet Service Provider.
Firewall / Modem Exceptions:
1- Disable SIP ALG setting
a. SIP ALG is a common feature found in most modems/routers which is detrimental and should be disabled; if you are unable to find the option in your device's interface then contact your Internet Service Provider for assistance.
b. Disabling SIP ALG resolves most issues, however, if you find that your phone is still not working properly then you may also need to open some ports:
2- Open the following ports:
a. TCP/UDP ports 5060-5065 (SIP) used for control and signaling