When reservists get put on active duty, how do they make up the pay difference?

Read ar 37 104 4 chapter 33

The military can provide replacement income, but this is only for "extended and frequent" calls to active duty - Title 37 US Code Section 910. Currently expires Dec 31, but has been extended each year in the NDAA since it was originally enacted. No guarantees, however....

§910. Replacement of lost income: involuntarily mobilized reserve component members subject to extended and frequent active duty service
(a) Payment Required.-The Secretary concerned shall pay to an eligible member of a reserve component of the armed forces an amount equal to the monthly active-duty income differential of the member, as determined by the Secretary, when the total monthly military compensation of the member is less than the average monthly civilian income of the member. The payments shall be made on a monthly basis.
(b) Eligibility.-
(1) A member of a reserve component is entitled to a payment under this section for any full month of active duty of the member, when the total monthly military compensation of the member is less than the average monthly civilian income of the member, while the member is on active duty under an involuntary mobilization order, following the date on which the member-
(A) completes 547 continuous days of service on active duty under an involuntary mobilization order;
(B) completes 730 cumulative days on active duty under an involuntary mobilization order during the previous 1,826 days; or
(C) is involuntarily mobilized for service on active duty for a period of 180 days or more within 180 days after the date of the member's separation from a previous period of active duty for a period of 180 days or more.
(2) The entitlement of a member of a reserve component to a payment under this section also shall commence or, if previously commenced under paragraph (1), shall continue if the member-
(A) satisfies the required number of days on active duty specified in subparagraph (A) or (B) of paragraph (1) or was involuntarily mobilized as provided in subparagraph (C) of such paragraph; and
(B) is retained on active duty under subparagraph (A) or (B) of section 12301(h)(1) of title 10 because of an injury or illness incurred or aggravated while the member was assigned to duty in an area for which special pay under section 310 of this title is available.
(3) A civilian employee of the Federal Government who is also a member of a reserve component is not entitled to a payment under this section for any period for which the employee is entitled to-
(A) a differential payment under section 5538 of title 5; or
(B) a comparable benefit under an administratively established program for civilian employees absent from a position of employment with the Federal Government in order to perform active duty in the uniformed services.
(c) Minimum and Maximum Payment Amounts.-
(1) A payment under this section shall be made to a member for a month only if the amount of the monthly active-duty income differential for the month is greater than $50.
(2) Notwithstanding the amount determined under subsection (d) for a member for a month, the monthly payment to a member under this section may not exceed $3,000.
(d) Monthly Active-Duty Income Differential.-For purposes of this section, the monthly active-duty income differential of a member is the difference between-
(1) the average monthly civilian income of the member; and
(2) the member's total monthly military compensation.
(e) Definitions.-In this section:
(1) The term "average monthly civilian income", with respect to a member of a reserve component, means the amount, determined by the Secretary concerned, of the earned income of the member for either the 12 months preceding the member's mobilization or the 12 months covered by the member's most recent Federal income tax filing, divided by 12.
(2) The term "total monthly military compensation" means the amount, computed on a monthly basis, of the sum of-
(A) the amount of the regular military compensation (RMC) of the member; and
(B) any amount of special pay or incentive pay and any allowance (other than an allowance included in regular military compensation) that is paid to the member on a monthly basis.
(f) Regulations.-This section shall be administered under regulations to be prescribed by the Secretary of Defense.
(g) Termination.-No payment shall be made to a member under this section for months beginning after December 31, 2016, unless the entitlement of the member to payments under this section commenced on or before that date.

I was fortunate enough during my reserve time to not have to deal with this, but I don't think there is much in the way of help, other than possibly explaining the situation to creditors, and asking them to suspend payments, or make interest only payments, while you're activated.

If you get activated and are married and have rent or mortgage you will receive separation pay, housing allowance, depending on where you are sent there is also immenent danger pay. When I was activated in 91 I made more in my civilian job. but what I was getting while activated I did very well and you only get taxed on base pay

I don't have that problem. Active duty E-5 with 5 years TIG, plus with BAH makes about half again more than I do per year.

My civilian employer makes up the difference. They do not have to though.

Some places that you are dealing with might have lower payment plans, it would take longer to pay off the debts.

In most cases, there is no way to make up the difference. There are some programs available if you are a federal or state employee, but there are some very specific rules that may not apply to you. Mainly only large companies will provide 'differential pay' and only a few will maintain your current insurance or retirement. You can qualify for some programs to lower your credit card interest, get out of a lease, lower or defer your student loan payments, but in general you don't make up the pay difference. BAH for a CONUS assignment might only be 55% or lower. BAS, Family separation pay, combat pay, etc., might not apply or be lower than expected. I went from a federal job as a firefighter making $60,000 a year plus hazard pay and overtime to an E-5 at Fort Hood where my net was around $21,000. My wife and kids moved in with her Mom, I sold everything I could and lived on discount food and didn't go out or do anything for 90% of the mobilization. I sold plasma every week and did odd jobs on weekends for extra money, went to the monthly 'pay day' dinners at the local VFW and on post, and lived in a shitty little apartment with no phone, internet or TV, all to save money. This is why active duty or activated soldiers so often qualify for food stamps, WIC assistance or other programs. If you are single, can play the system, and live on a budget, you can survive, but it is tough.

Unfortunately there is no additional resource fir monies other than the BAS and BAH that you may qualify to receive. Check into how much those tax free allowances my pay before you file for food stamps.

Find an employer that pays the difference as a benefit, save up to be able to ride the financial storm when you go active, or live life aso if you are making that much less and invest the rest.