An important task of testing a telecommunication protocol consists in analysing logs. The goal of log analysis is to check that the timing and the content of transmitted messages comply with specification. In order to perform such checks, protocols can be described using a constraint modelling language. In this paper we focus on a complex protocol where some messages can be delayed. Simply introducing variables for possible delays for all messages in the constraint model can drastically increase the complexity of the problem. However, some delays can be calculated, but this calculation is difficult to do by hand and to justify. We present an industrial application of the Coq proof assistant to prove a property of a 4G protocol and validate a constraint model. By using interactive theorem proving we derived constraints for message delays of the protocol and found missing constraints in the initial model.