I'm in a meeting; if it's urgent please WeChat me.
Tips
grammar
在 + verb forms the progressive aspect ('be -ing'). 我在开会 = I + currently + hold-meeting. Same pattern as 我在吃饭 ('I'm eating') and 我在工作 ('I'm working').
register
Universal soft brush-off in Chinese workplace texting and calls — even when the meeting is loosely defined. Like English 'I'm tied up right now', it asserts 'I'll get back to you' without committing to when.
