Comments on: Best Windows Mobile SIP VOIP Clients http://cheapestvoipcalls.net/best-windows-mobile-sip-voip-clients/ Sun, 29 Aug 2010 15:51:56 -0700 http://wordpress.org/?v=3042 hourly 1 By: Rob http://cheapestvoipcalls.net/best-windows-mobile-sip-voip-clients/comment-page-1/#comment-37 Rob Tue, 05 Jan 2010 10:16:46 +0000 http://cheapestvoipcalls.net/?p=305#comment-37 Regrettably - SIP was never designed to be Network Address translation (NAT) friendly and as 99% of cell operators use NAT and dont offer SIP proxys at their boundary firewalls / gateways SIP can be very limited via Mobile. for example...get two SIP enabled windows mobiles behind a common NAT firewall and registered with an external SIP proxy. The signalling works first time and the call alert will go through... sadly though in the majority of cases the voice path will not connect - why ? because the voice part of the SIP call is based on RTP (real time protocol) is seen as an "unsolicitied request" from outside the firewall - therefore the firewall doesnt know who to forward it to. Do you see many cell operators adding SIP proxies ? when it may cause them to loose call revenue ?...no?....me neither !! ;-) Praise be to IAX2 protocol and asterisk! - and to ESCsoftphone / voipsurfer (Same product different badge) for making it available on WM based devices Regrettably – SIP was never designed to be Network Address translation (NAT) friendly and as 99% of cell operators use NAT and dont offer SIP proxys at their boundary firewalls / gateways SIP can be very limited via Mobile.

for example…get two SIP enabled windows mobiles behind a common NAT firewall and registered with an external SIP proxy.

The signalling works first time and the call alert will go through…

sadly though in the majority of cases the voice path will not connect – why ? because the voice part of the SIP call is based on RTP (real time protocol) is seen as an “unsolicitied request” from outside the firewall – therefore the firewall doesnt know who to forward it to.

Do you see many cell operators adding SIP proxies ? when it may cause them to loose call revenue ?…no?….me neither !! ;-)

Praise be to IAX2 protocol and asterisk! – and to ESCsoftphone / voipsurfer (Same product different badge) for making it available on WM based devices

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