[memling]

ASN.1 standards are free: https://www.itu.int/rec/t-rec-x.680/en

Many, though not all, specifications that use ASN.1 are also freely available. I've been out of telecom for awhile, so I don't know the status of the newer standards, but when I was working in the business GSM MAP and MMS were the only proprietary ones that were an issue.

[userbinator]

GSM standards are also freely available --- look at 3gpp.org or etsi.org --- the biggest problem is finding which ones actually contain what you're looking for.