"He's busy with me~"💦
I like to imagine that, given Ichigo's overall popularity with both men and women, he definitely has quite a few fans. Some that maybe don't stay as secret admirers for long. Who maybe try to chat him up and flirt and shoot their shot. Perhaps a newbie shinigami trying to get Ichigo to invite them out without really getting the hints that Ichigo is not interested (and very much secretly engaged with a recently 'allied' ex-espada)
And Grimmjow is proud of having Ichigo all to himself, but also... very possessive. 😂💕
So perhaps he gets tired of the texts and decides to send them something they can't misunderstand, something along the lines of "he can't answer right now, he's busy fucking me~ 😏" lmaoooo All while Ichigo watches in reluctant amusement; it's going to start a mess in the seireitei... but it's not like he can stop Grimmjow right now when he's still in cloud nine and waiting for Grimmjow to get so worked up he'll ride him until Ichigo can't see.🤭
