Wirelessly posted (Opera/9.50 (J2ME/MIDP; Opera Mini/4.0.10031/230; U; en))
I have found the the voice quality is diminished when TeleNav is running in the background. If I must take the call, I'll use the Application Switch tool, close out TeleNav, and then return to my call. It's lame, in my opinion, that TeleNav doesn't automatically suspend itself and then let you resume after the call. If I remember right, previous versions did allow for this.
To answer the rest of your question, after you end your call, go back into TeleNav and the location that you had found directions to will be in your Recent list and you can get directions from your current location.
I hope that helps.