actually, you should speak 1 as yi1 according Chinese grammer, but in daily life, we speak 119 ( yao1 yao1 jiu3), 114(yao1 yao1 si4)...the reason is the frequency of (i) is rather lower than (ao), people cann't hear yi1 very clearly, especially in the war time.

Hi Jennifer ,it is common in chinese people daily life and it is a encrusted with tradition.there are some reasons,i hope it can give you some help. 1. in china 幺（yao)means the youngest , 1 and 幺 has the same Pronunciation.i guess it is the one reason for us can accpet 1(yi) as （yao) 2.once yi as yao appeared in some military phrases,especially in phone numbers,coz the Pronunciation 1(yi) and 7(qi) are similiar and easily to be confused if we read fast.

I have done a research 这种特殊的数字发音，起源于战争年代。汉语韵母的频率。据研究发现，在汉语韵母中“i"的频率最低，清晰度最差，跟"a"比，"i"的功率只是"a"的一半，差了至少3 分贝。大家知道，战场上噪音很大，通信兵如果发“i"对方可能听不清楚。通信不畅，在生活中尚且会造成许多麻烦，若发生在战场上，可能要付出鲜血和生命的代价。于是大家想出一个好办法，全面替换"i"音，因汉语里"幺"指小的意思，所以把"yi"读做"yao"。http://bbs.tiexue.net/post2_1442933_1.html 现在一般读一串数字时会读成yao，单个数字一般读yi

Hi KK/Jennifer, I'm learning chinese and what I discovered is that "yi" and "yao" both refer to number "one" (same character '一'). "yao" for instance is preferably used when giving your home address or telephone numbers (instead of saying "yi san liu" as to 136, you can say "yao san liu")