TD-CDMA (time division code division multiple access) systems adopt a hybrid access scheme. Using different spreading codes several radio carriers are defined on the same bandwidth. On each carrier a time frame is used to multiplex different channels. So a channel is defined by a code and a time slot. This is the scheme adopted by UTRA-TDD (UMTS terrestrial radio access-time division duplexing) interface and the TD-SCDMA (time division-synchronous code division multiple access) system standardized by CWTS (China Wireless Telecommunication Standard). As with pure CDMA schemes, the call admission control (CAC) mechanism must guarantee the quality of all active calls in terms of signal-to-interference ratio (SIR), limiting the accepted traffic in the system. However, the problem of admission control is strictly related to slot assignment scheme since different interference conditions can be experienced on different time slots. In this paper we compare the performance of two CAC mechanisms for voice services under both uniform and non-uniform traffic conditions considering different slot assignment schemes. One is based on the number of active calls (connection based CAC) and one on run-time measures of the total power emitted in the downlink and received in the uplink by the BS (power based CAC). We also consider an ideal method (ideal CAC), which provides the upper bound of the capacity.