The issue is not that I mind being on call when things are broken. The issue is that they want me to pay my own way in order to be on call. It's insulting.
Ah. Well in a salaried world I would say fair enough (my organisation pays for phone and internet connections for on-call staff).
In a startup world... I don't know. But either way, I don't think there's any point in feeling insulted. It's just a matter of fact issue of "I choose not to spend money for an Internet connection. If the company pays for it, I will do my best to attend after-hour calls". No point bringing emotions into it.
But you know, if you have a stake in this company succeeding and if the company has no money... if it was me, there are definitely circumstances in which I would pay for it.
To be honest, I can see why you'd feel insulted.