H and I are trying to reconcile, but I don't really believe he has ceased his relationship, at least via telephone, with OW. How can I be assured he has? This has been an off-and-on affair for six years, of which he really only admitted about 6 months. I really would like to save our marriage, but I am always so suspicious. Any ideas? Thanks.

p-anne, You are to be present when he sends NC letter and it has to be specific to her that she is not to contact in any form. No emails, phone calls, or face to face contact. In Other words, he wants her totally out of his life!
If the phones, he's to tell her no more and hang up. If she writes, he's not to answer her. If she approaches in person, ignore and keep going.
If necessary, and she stalks him, get a restraining order!
To put it bluntly she is to be considered in the past tense! AS thought dead!
As for you, you have to be able to check phone calls, even cell phone bill, and entitled to his email addresses and his password. He may have secret email on other sites. Most are free. So you may even have to install spyward on your computer to see if he's doing this. You do not tell him you have it on computer.
If he refuses to give his password, screennames, and allow you to check cell phone bills, etc. Then he has something to hide. AS Dr.Phil says, he who has nothing to hide, hides nothing!
The Wayward spouse it the one who is to write and send the No contact letter. And you should see that it is sent with your own eyes!
HOpe this helps you. God bless, LouLou