Looking at the "services" they offer on their website and working in telecoms I wouldn't say they look that great.
Their headline statement seems to indicate they specialise in "purchasing, procurement and commercial management of telecommunications" isn't this similar to what phones4u do?
As for their advertised expertise in M&A, doubt I will be engaging them in the next deal I manage. Not quite sure how shareholders would react to "well I decided not to use a big 4 firm this time, instead I have gone for Source8" other than the fact they would probably fire me.
That said their client list looks ok, apart from the fact a number of the pages are under construction, but if you are after a career in procurement could be a safe bet.
However if you want to get involved in true telecoms consulting look elsewhere (e.g. PWC, Spectrum, PA, Analysys Mason, Ovum etc) as these are the firms that actually advise telecoms firms at a strategic and/or operational level.