Add a stakeholder on a Professional plan

I noticed that in my pagerduty screen I have the possibility to add up to 5000 stakeholders. Looking at the documentation I saw that this would only be possible with the business plan.

It turns out that I need this function to notify a stakeholder via phone and SMS when they are subscribed to an incident. Is this possible on my current Professional plan? What would be the cost if I registered a new Stakeholder user?