Re: Considering it's $1m...
Even without messing with the phones, so long as the attackers only made minor tweaks to the emails in each direction I doubt a phone call would help. Calling to confirm that the payment details have been sent or that payment has been made wouldn't help, since each side had received appropriate emails. To spot it each side would need to read out their email's to the other side, most importantly including the exact bank details, to confirm what they'd sent/received matched, but that seems unlikely to happen, especially when for most scams like this (at least until now!) a simple "did you send me this email?" is enough to spot the fake.
