I'm not ordinarily a fan of per-seat pricing but I think it makes sense in this environment, as software adoption will be top-down, the sales process is high-touch, they can build it into per-employee overhead, etc. I'd caution you that, as you likely know, employee turnover in this industry is
insane and as a result of that and standard staffing practices the seats in use are going to fluctuate constantly. This isn't totally dissimilar to CS, so you might look at what ZenDesk and the ilk do with regards to floating seats, etc.
If you're really getting X0% to Y00% in per-collector productivity then I say start by asking for for $100 per seat per month, if you're wedded to per-seat pricing. You can price anchor it around "less than a single collector-day" and that leaves plenty on the table for your clients. I'd then start walking the price up (if it is worth $100 a collector it is probably worth $150/$200/$250) until you start losing lots of sales on the pricing question.
If you've got very heterogenous clients number of people on the phones might not capture value as well. You might consider doing N tiers (maybe or maybe not publicly disclosed). I'd be thinking something along the lines of $500 for the mom&pop shops, $2.5k for "real businesses" doing debt collection, and $10k++ custom pricing for large/enterprise accounts. You could have multiple segmentation levers between plans, such as e.g. size of the average receivable, total volume of receivables, type of collection, what data sources you pull in to do the calculations ("Pay more get more!"), etc.
The great thing about high-touch sales is that if you hate how your pricing strategy interacts with any one client you just change it for the next one.
If you'd like to discuss this in more detail non-publicly, email me. I have a weird personal interest in this field and am happy to trade informal consulting on SaaS pricing for war stories.