My voip provider supports two calls per line.  When I'm on a call and a new call on the same line comes in, if I put the first caller on hold to answer the second call, then try to go back to the first call, I can hear them but they can't hear me.  So basically I can never continue my conversation with the first caller.  I have tried placing the first caller on hold using the HOLD button, but no luck, and I have tried using the HOOK to change callers, but no luck there either.  Does anyone know why I might be having this problem and what I can do to correct it?

SPA504G software v. 7.5.2

On another note, I only have 3 of the 4 lines registered and I wonder why the phone doesn't "roll-over" to the empty line when a second call comes in, instead of ringing the line already in use?