Likely either from yours.. or maybe someone elses, there has been a mass amount of phone messages...
(say someone was getting harrased and getting a message ever 3 mins.. causing an email every 3 mins from that address) it could have triggered something in the spam filter, to now start blocking it.
A few things to help it hopefully not catch it.
Adding to the address book, will help usually white list it.
(but i am not sure.. if the address changes each time, based on the number calling you? address book really wouldnt help)
In the spam folder, mark each of them as NOT SPAM and have it move them back to the inbox.. this should help the spam filter learn, but it can take quite a while.
BEST solution.. Click the little cog wheel in the top right of the webmail.. then go to settings.
Go to FILTERS, then ADD
Filter name call whatever you want.
Under SENDER, choose either CONTAINS or ENDS WITH and put in @vonage.com (or whatever the end of the email addresses is), then at the bottom tell it where you want them delivered (your inbox likely).
This will force stuff that went into spam, into your inbox anyways.
(i used this when i used my rogers email for my BLIZZARD stuff. Due to so many FAKE spam phishing emials, blizzard addresses were being put into spam. Doing this, forced all (including the bad ones) back into my inbox)