GSM通話流程
https://wenku.baidu.com/view/79cd8ae59b89680203d82552.html
https://wenku.baidu.com/view/84d99f34ee06eff9aef80726.html?sxts=1585734030360
https://wenku.baidu.com/view/56555403eff9aef8941e0662.html
http://www.manongjc.com/article/67530.html