i am working as voice and accent and softskills trainer for the past 2 years in a international call center in delhi.
i personally feel if one wants to learn english, spoken or written british council is the best place to go.
as far as american voice and accent is concerned one needs a lot of practice with consonant and vowel sounds,syllable,syllable stress,pronunciation,intonation and liasioning of words.
one can search on net for these topics and take help from people working in call centers.
please feel free to ask any question related to v&a and softskills.