Learn Dutch: Hij belt mij vaak. - He calls me often.

Both mij and me mean "me". mij is the stressed form, used when you want to emphasize who is being called (He calls me often, not someone else). If you don't need to add that special emphasis, you can also just say Hij belt me vaak.

What is the full verb for belt?

The infinitive (dictionary form) is bellen, which means "to call" (by phone) or "to ring". For Hij (he), we take the stem (bel) and add a -t, giving us belt.

Could I say Hij vaak belt mij like we often do in English?

No, because in a normal Dutch sentence, the working verb must be in the second position. Hij is in position one, so the verb belt must come right after it. The adverb vaak (often) usually goes after the object.