I do not have a smart phone, I have a dumb phone that can send SMS messages. In the northeast where I have done all of my SOTA activation I have almost always found old-fashioned cell service available and SMS spotting works. RBN almost always seems to find me, also, but it requires having posted an alert (for me, without a smart phone, that must be done before I leave home).
Three hints: since a non-smart phone doesn’t have a real keyboard, composing an SMS spot in the field is difficult, especially in bright sunlight. I precompose draft SMS spot messages beforehand, and I save messages after sending so I can edit the frequency and send again when I want to QSY. The second hint is, don’t be impatient. The sotawatch site refreshes every five minutes, so don’t expect an immediate response when you call CQ, keep calling for a while before giving up and moving to another band.
The third hint is for chasers at home, if you expect someone to show up, use the main RBN site to see his exact frequency when he starts calling CQ and how strong is signal is at various locations.