Even with quarterly I'd expect the billing date to be the reset date & then the same date every month. So the number of actual days you have to spread the minutes over will vary with month length.
Just a thought but the £7 pm you're paying for 700 mins would almost cover the £8 pm cost of a mobile SIM with unlimited minutes. If you have good mobile signal indoors a cheap handset would do the job. Otherwise you'd need one that supports WiFi calling.
