Beware! He could very well be telling the truth. If it's a phone you're talking about I once got a huge bill for the same sort of thing. I didn't even accuse my husband because I know he wouldn't do that. It turned out someone simply decided to use our phone # and put everything on our bill. I phoned "Telus" our telephone company in British Columbia, and they checked it out and took it off my bill. If it's the web, your husband (or you) could have given out info that other people could use and basically do the same thing that would happen on your phone bill.
Check your desktop on your computer and or "documents" and see if he has saved any info to keep contacting these type of chatlines. If you have kids in the family they could well be doing this too. Sex is the easiest thing to bring up on a computer whether you go hunting for these sites or not.
I'd give your husband a break. People using other people's phone numbers or email accounts for this sort of thing is nothing new. Report it to your phone company or your server and get it straightened out.
Good luck Marcy