Can't you buy a dual sim phone and have one local sim and your old sim? I've had my number out here in Thailand for 10 years on EE without issue but I only turn it on in the phone when I actually need to use it. That way, I guess it doesn't look like continuous usage but they still send me almost daily texts telling me it's 1.80 a minute. I also buy 10quid of credit for it every size months or so just to keep it topped up.