Long Term Evolution (LTE) is a totally IP based cellular network, that only supports packet switching (PS) function, previous cellular networks, such as 2G and 3G, were basically designed to carry voice calls through circuit switching (CS) service, then data service was supported through techniques that basically encapsulated data inside the voice-call connections. In LTE there are many alternatives for offering voice services over LTE to ensure the service continuity over celluar networks, these alternatives include Voice over LTE (VoLTE), also include Over the Top Voice over IP (OTT VoIP) one of the proposed alternatives for voice service over LTE, which is the lowest in cost and complexity. The objective of this paper is, to investigate the quality of the low cost OTT VoIP voice service, as an alternative to VoLTE, especially for those Mobile Network Operators (MNOs), who aren't ready to deploy the highly complex and costly IP Multimedia Subsystem (IMS) infrastructure of VoLTE . Performance evaluation for OTT VoIP over LTE, compared with VoLTE is provided, from the most important QoS parameters point of view, in different scenarios with different number of users assuming congested and non-congested network states, the analysis for all these scenarios are based on simulations using OPNET 17.5 simulation tool, the results provide a performance template for MNOs who doesn't want to deploy IMS and doesn't have legacy 2G/3G networks.