DSP Topic for Newcomers
I think the  most fundamental things that anybody trying to understand DSP should know are:


Nyqvist Sampling Theorem and Effects of Aliasing
Need for DFT and the use of FFT to achieve the same.
Basics of Digital Filters.


You have mentioned the audience as telecommunication engineers. So you could also add a lecture on the  need for modulation,some basic modulation and demodulation techniques and need for interpolation and decimation.