For what it's worth, backdoors have been discovered in Ericsson switches, see [1], it's fascinating reading. It's unclear who put them there. Wikipedia [2] alleges that the US embassy in Athens was behind the telephone intercepts. Naturally, Wikipedia is n o t a reliable source in such matters.
I used to work in telecoms myself about 20 years ago, and at least at the time, much of infrastructure was insanely insecure -- I could have backdoored much of the internet's backbone as a lone programmer had I wanted to: our software was used by most big telcos. Not once was our code audited (at least openly). I very much hope that security is taken more seriously now.
