Apex / RMA question

KM_1

So I have moved abroad and thought I had no balance on my credit card but my old address forwarded a letter from Apex which asked me to contact them. I think it can only be my credit card as I had a small balance with Natwest at one point that I thought I had cleared. The time difference means I can not call them in their opening times so I sent them a nice letter explaining if they could contact me by mail I will be happy to discuss the matter with them. I think I only owe a small amount (200 approx) and want to pay back the debt. The house I was staying in before I left the UK now get several calls a day from a company from RNA or RMA, are they related to Apex in some way? They have told them I dont live there any more but they wont stop calling. I am reluctant to give them my phone number as I do not want them calling me in the middle of the night here but I do want to sort this out. Is there a way I can stop them calling my old house and harassing them? I have not had a reply to the letter I sent to Apex as yet and if I gave them my address why do they persist in calling them in the UK when I no longer live there and when I have given them my new address.