It’s unlimited, and you can add as many as you want. Just make sure your current plan covers the total extensions/users for all PBXes/Voice platforms.
No, trunks and channels do not count as users and have no impact on cost.
Yes, it’s possible with an additional cost. For more info, click here
If your current plan has those features, no, but you should pay for each voice call alert or each SMS alert. Price per call and SMS are different based on receiver location. You should have an initial credit for SMS/Voice alert. For more info, contact sales
Yes, PBXDom offers an enterprise on-premises version for at least 5000 users and more, pricing starts from $60,000 annually; for more info click here
PBXDom plans are tier-based; they start from 48 users and continue by 64, 96, 128 up to 16,384 users. By entering your total extensions, it automatically offers the nearest plan that covers all of your extensions.
So, your total extensions equal 500 + 80 + 100 => 680. By entering 680 on the pricing page, the 768-user plan will be offered to you.