A telephone number is not an amount or quantity, It is a specific code made up of numbers, not unlike an serial number made up of numbers . If you don't say "你手机的 serial number 是多少", then you shouldn't say "你的电话是多少"
We always ask people " what is your phone number?" instead of "how many is your phone number"
The correct answer is "你的电话号码是什么" /Nide dianhua haoma shi shenme/
Also, Can I just say, "Nide dianhua shi duoshao?"
No, it means "你的电话是多少" (your telephone is how many), which doesn't make sense; If you say "你的电话是什么" people would understand you meant "你的电话号码是什么" (omitted 号码). But, grammatically 电话 /dianhua/ only means "telephone" , you need to add 号码 and say "电话号码" if you meant to ask "what is someone's phone number" instead of "what brand someone's telephone is"