"Riskbased" pricing is not far from what you suggest, so you are on the right track: Calculate call frequency and how much each call will cost you (time, resources). Divide it over your working time, and add a profit margin: Then you have a monthly fee for 24x7 support.

If you want, you could monitor individual frequency - maybe somebody uses your support more than others and would freeride on a fixed rate? Then you could price based on number of uses of your hotline pr. month, or software version (if some versions crash more than others).

This is how you do it in insurance, where "usage" is random, and each use (claim) will cost you money. If you want to know more about this, feel free to sit up a call, or send me a mail.

Best regards
Kenneth Wolstrup