Optotriac doesn't turn off
This is what you call triac abuse; a triac is not made for this kind of application. Like Steve says, triacs remain switched on on DC loads. You may have zero crossings where they normally would switch off, but I'm not sure how the triac reacts to very fast switching, like in your 56k EIA-232 signal. The zero crossing may be too fast to switch it off.  

I'd suggest an analog switch. Analog devices has zillions of them.