Will the line be busy by texting?

You mean, when you send a text message, will the phone not come in?

No, the text message is just a short moment. If a call comes in at this time, it will delay your mobile phone by one second or a few tenths of a second to show that a call has come in. As for writing short messages, I can't. Generally, writing short messages on mobile phones supports calling, and you can continue to write after answering.