نزيل
محتجز
I believe you can use
نزيل
if the institution is mentioned before. Then it would be implied that he is a prisoner even though you don't mention that openly.

نظام هاتفي مراقب للسجناء

نظام هاتف تحت المراقبة للسجناء

Background
All states but Texas provide telephone services for prison inmates. All local jails in Texas also provide phones for
inmates. These services increase state and local revenue through commissions, provide benefits to inmates and their
families, improve prisoner behavior, assist with rehabilitation and can provide valuable assistance to law
enforcement.

Advanced telephone technology has alleviated the security concerns of most states' prison administrators. In fact, new technology assists prison administrators and law enforcement
officials with crime investigation and prevention. Automated inmate calling systems allow prison officials to monitor all calls by inmates, track and cross-reference telephone numbers, generate reports for investigative purposes,establish individual levels of access, restrict the number and length of the calls, establish personal identification numbers (PINs) and authorize calling lists for inmates.1