How do I explain to my wife that I just don't like talking a long time on the phone without offending her?

I love my wife dearly but I don’t know how to make her understand that I can’t sit for long periods of time on the phone.. I just lose interest and then lo and behold I don’t sound interested so she gets upset. I can’t fake interest.. what do I do? I’ve told her how I feel but I can tell she really just wants me to be completely, totally interested in every word she says.