I was having an issue where 3cx windows clients could not connect to extensions with hardware phones. The phones are Yealink T26 handsets and are all on the same network with the 3cx clients. Here is the log: 24-Feb-2015 12:20:55.365 Leg L:261.2[Extn] is terminated: Cause: 408 Request Timeout/INVITE from local 24-Feb-2015 12:20:55.365 [CM503003]: Call(C:261): Call to <sip:firstname.lastname@example.org:5060> has failed; Cause: 408 Request Timeout/INVITE from local 24-Feb-2015 12:19:51.188 [CM503008]: Call(C:261): Call is terminated 24-Feb-2015 12:19:51.185 Leg L:261.1[Extn] is terminated: Cause: CANCEL from 192.168.1.125:64355 24-Feb-2015 12:19:38.777 [CM503025]: Call(C:261): Calling T:Extn:201@[Dev:sip:email@example.com:5062] for L:261.1[Extn] 24-Feb-2015 12:19:38.728 [CM503027]: Call(C:261): From: Extn:202 ("James Martin" <sip:firstname.lastname@example.org:5060>) to T:Extn:201@[Dev:sip:email@example.com:5062] 24-Feb-2015 12:19:38.728 [CM503004]: Call(C:261): Route 1: from L:261.1[Extn] to T:Extn:201@[Dev:sip:firstname.lastname@example.org:5062] 24-Feb-2015 12:19:38.726 [CM503001]: Call(C:261): Incoming call from Extn:202 to <sip:email@example.com:5060> 24-Feb-2015 12:19:38.725 NAT/ALG check:L:261.1[Extn] REQUEST 'INVITE' - basic check passed. No information for extended checks 24-Feb-2015 12:19:34.027 [CM504001]: Endpoint Extn:202: new contact is registered. Contact(s): [sip:firstname.lastname@example.org:64355 / 202] 24-Feb-2015 12:19:34.017 [CM504002]: Endpoint Extn:202: a contact is unregistered. Contact(s):  Hardware to software works fine, and hardware to hardware works fine. Following a hunch, I checked the PBX delivers audio box for the 3cx phone client extensions and they all can call the hardware phone fine now. Obviously, this is not an ideal solution. So why does the phone client not connect to the hardware phones without the PBX delivers audio box ticked?