Hey there,
Not a teacher
There isn't anything "incorrect" about the phrase, but it was a bit strange. I'd say
I tried to reach you, but your number was busy.
I tried to reach you by phone, but your number was busy.
**Confusing and unhelpful content deleted by moderator.**
I'd also be more specific about what the problem was with the phone number. Not to be pedantic, but I would also put a comma before the conjunction if the second clause were a complete clause (i.e. containing a subject and a verb).
I might also say,
I tried to call you, but the line was busy.
I tried to call you, but the line was out.
I tried to call you, but the number didn't work.
I tried to call you, but the number/line didn't connect.
Hope this helps!
Brad D