A little confused

Hi My Fiance and I are recently living together, and had some problems at the beginning with fighting and not getting along too well, but the past weekend was very nice, going down without a fight I think we might just make this work we should get to know each other a bit better as we're both used to living alone, plus I have a daughter and he doesn't have any kids. He loves her to bits but it's something to get used to living in one house with a two year old - he's actually doing great. Anyway - I had my share of relationship woes and the biggest problem for me was always trust. People lie and cheat, and I can't stand a lie even about this littlest thing - people should NEVER lie. Early in my relationship with my fiance, I caught him lying a couple of times, usually about going out drinking while he said he was home, but we resolved those issues and I will trust him again - it's just going to take a little time as he's destroyed my trust in him - but it's going better already. This weekend I caught myself snooping on his phone... I feel horrible but it's the trust thing coming in again. What I found though, is that a couple of months ago he rang up his ex-girlfriend, and she started texting him and thanking him for the call, and saying he was the "one that got away" and she wishes him all the best. His replies was very short and just wishing her well for the future as well, and they hadn't spoken since, but why would he have called her in the first place? This was after we got engaged already but before we moved in together... I'm a bit confused why a committed man would call up his ex for a chat ? And should it bother me at all? He has lied before, but never cheated on me.