Employment Verification Info Needed (2015, felons, company)

[coolguy2016]

Hello,
Do I need to mention Voluntary work on employment verification ? I had put that on my resume though (didn't mention exclusively on resume that it was Voluntary work)

Here is my story
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I have been offered a "Contracting" job opportunity and the Vendor company wants me to fill the employment and background check done which they will submit to Client.

My background is fine .. no felons etc ... but I have worked 1 yr as Voluntary (no paid) and have put it on my resume.
Now my question is ..

I worked
2008-2011 - I have all W2 etc for these times
2014-2015 - Worked Voluntary and have all documents (but no W2 since no pay)
2015-2016 - Worked as employee and have all the W2 etc

Now I have mentioned 2014-2015 exp - on my resume but didn't mention on resume that its Voluntary.

Now in my employment verification do I need to mention this voluntary work ?
Since I am afraid that if I don't mention it then it won't match my resume ...

Please let me know since I am really tensed and don't want to lose this opportunity inspite of having all documents.

Once again, thanks a lot and plz let me know.