I see it the opposite, if you're not paying a monthly fee then you have less security over potentially losing it. Its why I plan to go AAISP rather than sipgate, as while I've had a few sipgate accounts and they never deleted them even though I didn't use them for years, I'd consider anything I'm not paying a monthly fee for more at risk of deletion than something I am.
It would be worth finding out if this has ever happened to anyone? I think Sipgate are one of the oldest VOIP providers so if they do it, it would have happened to someone by now.