Definatly not a reliable indicator.
If they do re-innitiate contact there is the possibility they are wanting an ego boost, orbating, wanting attention, validation, already taken but stringing you along as an option and many other reasons.
I would normally apply the two strike rule. If they don't reply first time, try only once more.
The only reliable indicator of interest is if they show up on dates and make time for you. They could be calling and texting you 24/7, but if they come out with some BS each time you offer a date they most likely never will go on a date with you - this most commonly happens after the first date.