It depends what you mean by "learning telephony". Most telecom protocols are public, for mobile phones you can look at the 3GPP web site, for Wi-Fi look at IEEE 802.11, etc
But those are protocols, the hardware and software are proprietary so one outsider has no opportunity to learn anything. Patents are sometime very informative (but not so often).
And often the parameters of those hardware are known only by a few people, even in giant companies.
But there are open source telecom hardware websites. For example there are open source LTE base stations [0]. There are IMS implementations [2].
There are also innovative open source projects for example to create alternative networks based on Wi-Fi instead of LTE.
[0] https://bellard.org/lte/
[2] https://github.com/search?q=IMS+IP+multimedia+subsystem
