It very much depends on what country you’re going to be resident in. First of all, you’ll probably have to set up some sort of business entity in order to register with the appropriate tax authorities and social insurance agencies. You are generally considered to be working in whatever country you are physically present in while doing the work – with no regard for where your customers are located, nor where or how you are being paid for your services.
It shouldn’t be all that difficult to arrange for some sort of phone transfer (that’s how all those annoying call centers work, after all) – it will just be an additional business expense. It’s even possible using something as simple as Skype, though for a business you’d probably want a more professional service.