This problem usually occurs when an SPI firewall is enabled on the parent router.
The operators who had this problem reported that the registration works perfectly when they connect to the router via the LAN ports, the problem occurs only when they connect to the router wirelessly.
We have found out that in most cases the source of the problem is an SPI Firewall that is a built-in security feature in some routers. Most of the operators used an ADSL modem/router combo device and the problem was solved when they replaced that device with a standard ADSL modem and a single router.
Known routers that are affected: