Questions & Answers about wǒ yíhuìr gěi nǐ dǎ diànhuà.
This is because of a very common tone change rule for 一.
- 一 is normally first tone: yī
- But when it comes before a fourth-tone syllable, it changes to second tone: yí
- 会 in 会儿 is fourth tone: huì
- So 一会儿 is pronounced yíhuìr
This is called tone sandhi.
A few quick examples:
- 一个 → yí ge
- 一天 → yì tiān because 天 is first tone, not fourth
- 一起 → yìqǐ
So in this sentence, yíhuìr is exactly what you should expect.