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.
Wait, you think the Swedish secret service is going to coerce Ericsson into installing backdoors that let them spy in the UK?
I mean, sure, possible, but a bit further fetched than China doing exactly that, no?