If you find your phone needs to "wake up" before you can make calls, such as the first call fails but a second attempt will succeed, this may be caused by your firewall having a too short NAT timeout.
Try enabling NAT keep-alives on your SIP device, and set the frequency to 30 seconds. This will normally prevent most firewalls from timing out the SIP registration.
Alternatively, please refer to your firewall vendor for instructions on how to alter NAT timeouts.
Note: Handsets provisioned by Atlas will have NAT keep-alives enabled by default.
If you have an internet connection prone to packet loss or high amount of jitter, setting to TCP may improve success making calls or transferring calls. However, this will not help with voice quality if your internet connection is poor.