Linksys LRT224 routers might have issues with VoIP service. Here are some settings to be done in the admin interface to correct these issues:
Keep the firewall and SPI protection enabled. Review SIP ALG separately from the firewall: if it interferes with registration or calls, test disabling only SIP ALG. For VoIP-specific firewall rules, allow the SIP transport, server addresses and RTP media range required by your configured service; retain other rules needed for services such as DNS and time synchronization. Do not open arbitrary ports or disable access rules. Use the current device guide for firmware-specific controls. After changes, test registration, incoming and outgoing calls, and two-way audio; restore the prior configuration if testing fails.