It is lawful to inform through SMS or call about share market and its status to those clients who are dealing in the shares in ways compliant to Shariah i.e. they buy shares of companies which deal in halal items or which have their shares only in halal business, and they (shareholders) are not selling their shares before delivery rather sell them after delivery. It is unlawful to provide information of share market to those who sell shares before delivery or those who have shares in companies dealing in unlawful items. So you should not entertain such shareholders as your client.
Allah knows Best!
Darul Uloom Deoband