If this is for business purposes, it sounds more professional putting the subject of the sentence first. The word, occasionally, could also be used. Customers will occasionally contact us by phone for their queries. Customers will occasionally phone to ask questions. Customers will occasionally phone us if they have any questions. Hope this helps

Sometimes customers contact us by phone for their queries. Sometimes customers asks questions (on the) / (by) phone Sometimes customers phone us if they have any questions. Great work :)

The second sentence needs changing. 'by phone' or 'on the phone' is better