It depends on what your job is. If people rely heavily on written notes or e-mails, then writing would be very important, but if they rely more on talking on the phone or conversing face to face, speaking may be more important. Unfortunately, depending on what the job are, both might be equally important.

Work on improving your grammar and language skills in general. A small tip, if you are getting the job soon, try to write down some frequently used expressions in your job to have them ready to use in your speech on telephone or to write them down in your emails and meanwhile practice a lot,because it is really crucial to speak correctly with lesser grammatical mistakes. Many grammatical erros might render your speech incomprehensible and depending on the job's description as mentioned previously, you might end up not getting the job for favoring someone with better language skills. Good luck 加油：）

I reckon you can not say you understand if you do not know grammar. In English many things are communicated via tenses and sentenses.. Good luck!

Yes it is important. This site may be useful http://www.italki.com/knowledge/page/ENGLISH%20GRAMMAR.htm

Exactly! Depending on your job the importance of the grammar will be, but in general is a good idea to improve the grammar everytime that is possible, good luck!