$5 is a good price point for Stripe billing purposes, but 25c a call seems pretty high. I'm assuming that your backend is Plivo or Twillio or Voip.ms and the cost per minute is about a tenth of a cent.
I would consider doing 40 calls for $5 and 100 calls for $10.
Also, instead of just an affiliate program, consider doing social media marketing with call credits to encourage it. For instance, if the person retweets a message about callingly, grant them 5 call credits.
Also consider making the call page, like callingly.com/deftnerd, have the affiliate link within the "Get your free call page!" link so any users who sign up through my call page are associated with my affiliate account.
Another source of monitization is allowing for CNAME pages. Instead of callingly.com/deftnerd set things up to have deftnerd.callingly.com. For a $5 yearly fee, allow custom CNAME so I can set up call.deftnerd.com to CNAME to deftnerd.callingly.com. That yearly fee should also allow the user to remove any branding.
Lastly, if I'm in DND mode, the call page at callingly.com/deftnerd doesn't seem to indicate that. It would be nice to remove the "Call Adam Brown" button and replace it with a status indicator.
I built something similar a few months back, but it was purely Bitcoin related. Users would create an account and get a deposit address and as long as it had funds, the service would be live for them. It made sense when Bitcoin transaction fees were nearly free, but now they're as much as Stripe for sub $5 or $10 purchases.