What will collection agency do for disputed charge?Please help

My alarm company sent me a bill for a service they told me was free. Now they sent it to a collection agency. I filed a complaint with BBB but the company hasn't posted their response yet. I sent a letter to the collection agency telling them I'm disputing this charge. However, I got another letter yesterday urging me to pay and notifying me the approaching 30 day limit(in 2 days). It seems they haven't received my letter. I just sent it by regular mail but I have a copy of it. What should I do now/ Should I pay for it first and wait for BBB? Or can I just ignore the collection agency because I'm disputing it? I just don't want to ruin my credit. If the BBB says we have to pay, then I will. Otherwise, I think it's not fair for them to charge me. Please share your opinion. Thanks for help.