He's at work. Working. You are presumably also at work. Working.

Do you really need to talk to each other on the phone during the working day? Presumably you talk extensively during the evenings.

If we're being honest, unless you work at the same place, you aren't part of his work day. That's nothing to be surprised about: even if you were married and living together, you still wouldn't be part of his work day, and vice-versa.

Consider that some employers are also a bit funny about people using their personal phones in the workplace; whilst your employer may be totally okay with you sending personal messages regularly during the day, his employer may not.

Generally speaking, personal communication during the working day is usually reserved for emergencies.