Replace Device or Replace Firmware
This router is not recommended for use with VoIP services using the stock firmware from Linksys. This model router is known to damage and modify SIP packets in such a way that it prevents VoIP packets from being formatted correctly and can cause a multitude of problems.
You may be able to install custom firmware such as DD-WRT () as a replacement. Note that the procedure does require some technical knowledge and ability. The DD-WRT firmware allows you to disable SIP ALG and enable QOS (Quality of Service), but this has not been tested in our labs yet.