Internet Protocol Security (IPSec) must be allowed
Internet Protocol Security is a method of encrypting traffic sent through the Internet. It is used to provide a secure voice and data communication path. Some routers permit IPSec messages to be blocked. AT&T Wi-Fi Calling requires IPSec pass-through to be allowed.
Data ports must be open
Routers can be set to block traffic using certain ports. Ports 500, 4500, and 143, as shown in the table below, are used to communicate to the AT&T network and must be open. Port blocking is sometimes implemented in the form of access lists.
|Port||TCP or UDP||Service or Protocol Name||RFC||Service Name|
|143||TCP||Internet Message Access Protocol (IMAP)||3501||imap|
Maximum Transmission Unit (MTU)
The MTU represents the maximum packet size that can be transmitted. Wi-Fi Calling performs best if the parameter is set to 1500.