Negotiate for salary during 2nd call after verbal offer, but before written offer?

I was given a verbal offer Friday for a job I'm very excited for that comes with a nice (but not perfect) salary. I let them know that I'd think it over, and they told me they'd send the written offer in the next few days. Today they let me know that they need some additional information before sending the written offer, and now we have a second call scheduled for Monday afternoon.

I plan to try to negotiate salary slightly higher, but now that I have a second call before a written offer, I'm wondering if I should try to up the salary during the call (if they ask me), or wait until the written offer is in hand. Thoughts?

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