ok, before you go on calling yourself names and taking immediate fault, can you tell me how you asked her if she talked to him on the phone a lot? if you said it in a non-aggressive and quizzical way, then your girlfriend is trying to do a quick flip on you to make it about you and not about her. if you said it in an aggressive and accusatory way without giving her the opportunity to explain, my question to you would be, what's going on through your mind before you actually ask her the question? why are you immediately thinking that she is being unfaithful or lying to you? has she done something in the past that would make you doubt her?
if she has penpals, cool, but if she's also talking to these people on the phone, i would think that you should know about it, no? i mean, if my bf was penpals with girls all over the world, never told me that he talked to them on the phone before, and then slipped that information out without thinking during a conversation, i'd definitely ask him about it. if i asked him without any concern in my voice, there should be no reason for him to start attacking me. if i asked him in an accusatory way, implying that he has done something wrong, i would probably expect him to get a little defensive (especially if this accusatory way of asking him things is something i do all the time and he's fed up of my lack of trust in him).
i think it's normal to be slightly jealous. i think someone who has complete blind-faith in someone else is either naive or has been with them long enough to KNOW that they are trustworthy. my first boyfriend, who i dated in high school for 2 years, went to both proms with, and thought i "loved", had cheated on me. yeah, i was young and naive, but that relationship had an impact on my relations with other guys. my first instinct is to be on guard, to need reassurance all the time so that i know the guy is for real, to want to know girls that he talks to. over time, as i become closer with the guy, that instinct becomes less and less of an issue. my current boyfriend is pretty good about being upfront about stuff (most of the time haha). we have one issue that we still are working on, involving a girl from his past. it's something that shows its ugly face every 6 months or so, but would have probably not been as much of an issue if he had been more upfront about his relations with her (and if this girl had shown any interest in getting to know me).
as far as tolerating goes, i wouldn't like to date someone who is constantly accusing me of being dishonest or disloyal, especially when i've gone above and beyond what's normal to prove to him that he has nothing to worry. in order to avoid any kind of suspicion from my boyfriend, i'm pretty upfront with my relations with other people. i let him know what happened, who i talked to, how the conversation went so that he's updated and feels involved. if he ends up questioning something, i answer him. i don't mind a little bit of jealousy, as long as he gives me the opportunity to explain everything, because it shows me that he still cares
if he were to ask me in an accusatory fashion without even giving me the opportunity to explain, then i would get a little bit pissed. and if it was something that he did over and over again, it would tell me that he isn't making a conscious effort to be positive and i might call him a bunch of names and tell him he needs to grow up...