question about payment agreement

Okay--had a visa card that went to collection agency, wrote them right away and asked for payment plan. They were thrilledto hear from me and were more than happy to do a payment plan. It wasn't even set, they told me $100 orwhatever I could pay to start with and then if my situation improved, I could pay more. I've made three payments in a row every month and for the past two days, they have been calling my home. The person identifies himself as 'joe blow investigater'. I know if I called them back, I would know what they want but I don't deal with collection agencies over the phone. AND they know this cause in all my letters I tell them NO PHONE CALLS, written correspondence only. In fact, when the supervisor wrote me the very first letter accepting a payment arrangement, she said she was going to put me down for a C&D. so they're not suppose to be calling me now are they? Should I write them and ask them what they want and remind them that they're not suppose to be calling my home? maybe they're trying to find out if my situation has improved any, but the call was forceful and they wanted a call back TODAY--which they're not going to get. what are they going to do, sue me? we already have an arrangement. thanks for advice!! shirley