He should build his advantage on being the first and having a better product. Techical services he's using are not part of his core product.

..as tons of other startups do, and actually the only external services they use are Twilio and Stripe, others cited are just opensource frameworks/technologies (RoR, Bootstrap).

..and they are disposable. Most startups I see I know won't be around in 2 years.

they won't be around not because of the external services they've used but because they wont find the right business model. It's like saying that every ecommerce around will fail because they're using and external payment processor like Paypal. Maybe some of them can fail because of a Paypal "horror story" (account blocked etc..) but most of them will fail because they are not profitable.

I partially agree with you that relying on external services could be a risk, but in this case they've chosen services for some part that are not so easy to implement internally, and for that they've picked two new but highly growing services.

There already were competitors: http://presentnow.me

I'm going to have to disagree with you there. Neither Twilio nor Stripe are in the same business line as RollCallMe and thus are offering services, and thus not competing, therefore they have an incentive to keep prices down in order to attract more customers. I cannot speak for Stripe, but Twilio has lowered their rates in the past and does a wonderful job in regards to both technology and customer service.

Yes, there are no technical barriers to entry into this one-service industry, aside from knowing a Rails person who can wire up the app, but time-to-market, initial pricing and the potential for add-ons that can engage more users and augment the revenue streams give them the advantage. Choosing Twilio gives them that advantage.

Not a rock-solid app by any means, but one that's needed and doesn't compete directly against more mature and properly funded platforms.

Great idea. I wish them good luck.