I had a girlfriend that would do this, too.  
One day though the call dropped but I just kept talking. "Katie? Good, actually she's here right now... I'm sure she'd love to say hello but she's busy right now... No, mom, I'm not being difficult... BECAUSE SHE'S GIVING ME A BLOWJOB, OK?!"  
And that was they day I learned not to piss off the person whose mouth is on your cock.