Try pre booking
Sundays seem to be the most unpredictalbe of days. Either very slow or very busy. I imagine if a lady doesn't have something booked for it beforehand, she may be inclined to just take it off rather than sitting around waiting for a phone that may or may not ring.
Most ladies are able and willing to accommodate your preferred date or timing if they are given enough notice.
On the other hand, if your schedule is volitile, and you prefer to call last minute, make sure you leave a voice message with your name and number, as well as the date, time and duration you are hoping to meet up.
Most of us check our messages regularly (even when we aren't working) and you may be surprised to find that a lady who isn't typcially available on a Sunday may just have some free time and be in the mood after all!
Best wishes, and have fun!