When only one leg of a telephone call can be hard (either the remote party can hear your voice but you cannot hear theirs, or vice versa), the problem relates to your router device having trouble performing NAT (Network Address Translation). Below are some troubleshooting steps to follow:
1. Most IP telephones have support for NAT. This should be enabled, and is generally always labeled as "NAT Keepalive" on your telephone device.
2. Check to ensure that your router doesn't have any port forwarding for commonly used ports used in SIP or VoIP. This includes UDP ports 5060, 5061, and 10000-20000.
3. If your router includes a built-in firewall, try disabling it to conclude whether the firewall is blocking the traffic. If so, add the appropriate firewall rules to allow your voice traffic through your router. Additionally, if your router uses SPI (Stateful Packet Inspection) firewalling, it is recommended to turn this off, as SPI is known to have some complications dealing with SIP/VoIP.
4. If your router has an ALG (Application Layer Gateway) feature, try disabling this as well. While ALG could help in solving NAT related problems, the fact is that many routers' ALG implementations are wrong and break SIP.