I work at a job answering phones for hours. How can I serve G-d and gain reward while on the job? Just by thinking about Hashem?
The first way of serving Hashem while on the job is by doing the job properly, with full attention on answering the calls in the best possible manner, yet with the additional intention of doing a mitzvah in working faithfully.
This idea is quoted from Rav Yisrael Salanter concerning a famous Midrash (Talpios), which states that Chanoch used to stitch up shoes, and with every stitch used to think lofty thoughts for the sake of Hashem. Rav Yisrael explains that the thoughts were not unearthly, kabbalistic thoughts about Hashem, but rather thoughts of doing the job properly (every single stitch in place), so that the shoes would be strong and the owner would reap full benefit from them. This, too, should be the intention of each person on his job: his focus is on his work, on performing the job faithfully and giving clients the best possible service. At the same time, this itself is done with Hashem in mind. The intention of Rav Yisrael to sow good stitches is termed by the Midrash “intentions on the unity of Hashem.”
These thoughts themselves will bring you to think of Hashem, of how He gave you the job, of how He directs your life, and so on, and each thought about such matters fulfills a mitzvah. Certainly, your thoughts should not drift to the extent that the quality of your work would be affected.