Abstract: Background Patients in Belgium needing out-of-hours care have two options: the emergency department or the general practitioner on call often organised in a general practitioner cooperative. Currently, there is no triage system in Belgium so patients do not know where to go. Methods Our primary objective was to examine the ability of a newly developed telephone guideline, called 1733, to adequately estimate the urgency of health problems presented by simulated patients. Ten clinical vignettes were presented to 12 operators in a simulated phone call. The operators had to assign a protocol, urgency level and resource to dispatch (ambulance, general practitioner house visit, etc.) to each case. Results A total of 120 phone calls were analysed. The operators chose the right protocol in 69% and the correct urgency level in 35% of the cases. The proportion of under- and over-triage was 26% and 39%, respectively. There was important variation in between the operators. The sensitivity for detecting highly urgent cases was 0.42, the specificity 0.92. Conclusion Using the new Belgian 1733 guideline for telephone triage, operators mostly chose the appropriate protocol but only chose the correct urgency in one out of three cases. In this phase of development, the studied telephone guideline is not ready for implementation.