Apology Letter for Delayed Response

Dear {Recipient},

I'm sorry for not texting back right away {last night/yesterday/etc.}. Please know that I would never ignore you on purpose. I had simply {forgotten to check my phone/fallen asleep/etc.} and read your message just now.

For urgent matters, please call instead of texting. You may also reach me at {landline number}.

Best,
{Your Name}

Hello