On Earth - one of the most common methods to call people to worship (at least in our Western or European based cultures) is the use of bells. Church bells are so common that most people think first of churches when they hear bells or of bells. This might be those of us old enough to have grown up hearing church bells; you younger snots may not. Probably an urban vs. rural thing too.
But bells are expensive - no really expensive! And just because they are what were most common on our world, are they right for your high fantasy world? I like to think that different kinds of religions would use different methods to call the worshipers to prayer or to reminded them of their religious obligations.
So what am I thinking? Drums carry for decent distances - drums may be more appropriate for religions that are more in tune (no pun intended) with hunting or animals and less with metallurgy. What about trumpets? Their sound carries too. It probably takes two trumpeters, at least to make a cool impression; where as one old ugly guy could probably ring the bells. Trumpets may be a flashier version of bells for money and trade oriented religions. Plus, bells are a huge upfront investment. Trumpets would be cheaper up front but more expensive day-to day. What about smoke or incense? This might be more difficult to sense immediately, but it the person knew to keep their eye open for it, it could probably serve as a proper signal. Maybe grey smoke is a warning that services are about to start, followed by blue smoke (the five minute warning) and then steam as they put the fire out as services start (now you know to run). I am thinking that putting some manner of incense or other scent in the smoke would help catch people’s attention, but in big cities, the stench from medieval life must have been overwhelming, so it may be unrealistic to think they’d be able to smell the incense over the manure in the gutter. I think smoke signals work for some fire or sun related religions, or even religions of the dead, but that’s probably because I use to do the incense at funerals. Then again, sun worshipers might use mirrors to reflect light all over the town.
But this is high fantasy! Do the magical religions shoot fireballs or lightning bolts into the air? Do the mentalists send out a massive wave of telepathic messaging? Does the captive dragon get poked in the butt so he screams out his call? Especially if your game has all the priests pegged as spell casters, maybe they have special spells. It would be pretty cool if the priest walked out of his temple ten minutes before services and threw an aurora borealis into the sky.
We always seem to come to this question: Why does it matter? Fantasy worlds have hundreds of gods. (If you need more, check out Gods and Demons!) Each god and perhaps even each religion needs to be different from the others. You can’t have the services for the war god and the magic god be the same as a Judeo Christian service. Since you aren’t going to have your players attend services for other gods, then the way that the faithful are called to worship will be how they perceive these religions.
OK - You need something more combat related? If you know this stuff - You can write missions. The goddess of beauty has a series of musical bells summon the worshipers, but someone stole the bells - They must be retrieved before the major festival next week. The sun god’s priests clear the skies above the temple so they can shine the mirrors around and summon the faithful, but their spells are not working. The storm god’s priests are controlling the weather too and the two faiths are competing over sun vs clouds. Do the players get involved? Is a civil war brewing? (maybe just mob violence) Why now? Why did the storm god’s followers choose right now to mess with the sun god? Variety is not just the spice of life, but the spice of adventures as well!