If you are using a VigorTalk ATA, this problem is most likely caused by a compatibility problem with your ADSL modem or router.
The modem is not correctly translating the outgoing SIP messages leaving your network, thereby preventing you from making a call. The incoming SIP messages are not affected, allowing you to receive calls.
This problem usually occurs with ADSL modems that were manufactured prior to December 2004. The way to fix this problem is to upgrade the firmware (software) inside your ADSL modem. Please see your modem manufacturer’s web site for the latest upgrades, or speak to the manufacturer’s support staff for more information.
This is a know issue with NetComm NB1300 modems, some older D-Link and Netgear products, and any older products based on the Conexant chipset.