I plan on sending a DV letter to IC Systems but what account # do I put? It doesn't have a full acct# on my CR. Should I include my s.s instead?
No, don't give your SSN. Either just use the account number (partial as it looks on your CR) or get a CR directly from one of the CRAs...those usually show the whole account numbers. Third party pulls are formatted different and most of the time don't have the full account numbers and such.
With most of my DVs, I just put (example) 12345xxxx (this is all that shows on my credit report)...they all were able to respond to me and found the account.
ETA: If, by chance, they do come back and say they need more info to find your account, just give them the last 4 of your SSN..not the whole thing
In most cases, the debt collector has sent a prior dunning notice. If it is in your CR, they were required to have done so within 5 days of such reporting.
Dunning notice must advise you of how to DV. I would use no more to identify the account than what they have provided to you.