Unable to call other Cox digital phone user

Trying to figure out why approximately 95% of the time I cannot place a successful local call to another phone number who also happens to use Cox phone service.  I hear rings and then, "Your call cannot be completed...," or no ring and then a similar message.  It is seriously very rare that I successfully make the connection and the receiving person is able to answer AND hear me.  Sometimes, it is answered, I hear them say "hello," and I also hear continued ringing...they cannot hear me at all.  If I call other non-Cox service numbers, it works 99% of the time.  It just happens that the Cox digital phone service subscriber that I am unable to successfully call is a number I call most frequently.  That subscriber will then try to call me back if they heard the ring or answered it but were unable to hear me. Sometimes their outgoing call is successful, sometimes not.  Ironically, my Cox bill increased about $30 monthly, but the phone service is basically worthless to me this way.  Yet if I cancel it and just stay with internet, I am told it is better to bundle. I can bundle, but I want what I buy to work; it should be a rarity that problems with connectivity occur.   Anyone else have these phone issues?