Walkie Talkies (and I'm running off memory and may well be very wrong) seem to have limited range.
Even today they have terrible sound quality and might be risky for people trying to be stealthy
This article on pagers gives a quick overview on two-way pagers. (Link to a marginally more techincal article about one way pagers at the bottom of the one I linked)
Pagers were popular in the 90s, but was it possible not only to receive but also to send messages? Let’s figure it out.
debugger.medium.com
Your point about signal is a good one (and would sunnydale even have a pager signal tower?), but they're better than nothing for letting people to know [emergency, come to phone/try to call me].
(Also, note that pagers are inherently
very insecure/open communication tech)
Also, the year is relevant, as two way pagers are orders of magnitute more useful as someone can confirm recipt.
Buffy fandom wiki states season one takes place in 1996, so maybe a sending pager and a reciving pager might be needed.