I can make calls, but when the other person picks up, they can hear me but I can't hear them. I am connected to my Linksys WRT54G router and have Optimum Online as my internet connection. Is there any reason why this would be happening? I have tried the phone at another location and it works fine.
One way audio in most cases are routing issues.
Can you check if rtp packets are reaching both ways(There is no firewall blocking rtp traffic )
In cisco phones you could check that by pressing the "?" button.
have you checked NAT on your router? regarding RTP.
you might want to set up some static NAT rules inward that translate your public IP into the IP address of your SPA