I kind of had a similar problem.
His ex used to call/text him all the time and I was convinced she did it more often when she knew I was around. It used to really bug me that DP didn't brush her off or anything. In hindsight, I was being a bit irrational
So then he just started taking her calls when I wasn't around, which i wasn't too keen on either [is there a hypocritical emoticon?!] Then I realised, I justt had to try and get over the fact he had an ex who wouldn't leave him alone..
So the Ex issue....decide what you want- for them to be in contact but in 'secret', or for it be open?
In terms of taking phonecalls from anybody, as other posters have said, I reckon it depends on the context and how often you see each other.
So if you are just hanging out, then I don't think it is unreasonable for him (or you) to chat on the phone for a short while. If you are on a date or doing something in particular, then maybe you have a point. Try making a face at him when he is on the phone (in a jokey, nice girlfriend way) and see how he responds. He might not have realised that he was being a bit rude.....
good luck!