Calling them 'GSM' used to be fairly normal, at least in my experience. Think Nokia 3210-era. I still hear it being used every now and then, but it's not all too common anymore. We'll still know what you mean though.
As Simius said, 'mobiel' or 'mobieltje' is what's common now.
'Ik bel je op jouw mobiel' - 'I'll call you on your mobile'