A good excuse for not answering a phone call?

Well , its my bf's mobile call. He called me twice without me realizing.. and then texted like why are you not answering. One hour later he called me again and I just watched the screen without answering. I was just not in the mood to talk to him. As simple as that... because he gets really annoying at times and I need my space.
Im sure he's quite pissed off now.. but what should I do and offer as an excuse :/ ?
(Ps: this happened before and we stopped talking for a week...)
A good excuse for not answering a phone call?
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