Section 7. If any such company increases its capital stock, such new shares necessary to produce the amount of increased capital stock authorized shall, except as provided in the following section, be offered proportionately to its stockholders at not less than the market value thereof at the time of increase, to be determined by the department of telecommunications and cable, taking into account previous sales of stock of the corporation and other pertinent conditions, which determination shall be in writing and with the date thereof shall be certified to and recorded in the books of the corporation. The directors, upon the approval of such increase as provided in section four and the determination of the market value as hereinbefore provided, shall cause written notice of such increase to be given to each stockholder who was such at the date of the vote to increase, stating the amount of such increase, the number of shares or fractions of shares to which he, according to the proportionate number of his shares at the date of such vote, is entitled, the price at which he is entitled to take them, and fixing a time, not less than fifteen days after the date of such determination, within which he may subscribe for such additional stock. Each stockholder may, within the time limited, subscribe for his portion of such stock, which shall be paid for in cash before the issue of a certificate therefor.