The way I understand it is you bought shares of stock in XYZ company which say cost $10 a share and you bought 100 shares. Total cost is $1,000. However, you only had $600 in your broker account so, used margin (which is essentially a loan from your broker) for the $400 difference plus interest and fees. Next, you sell a call on your XYZ shares getting you a credit for say $50. XYZ stock goes down in value to $8 a share. Your stock holdings now are worth just $800 and even with the $50 credit, you only have $850. You broker now wants more monies from you and you get a margin call. The broker is only protecting their interest. Take note, you also, have a liability on your covered call since, if it is in the money at expiration, you would have to deliver the shares at the call price. Your solution is put up more monies to get rid of the margin call. Then, sell your call option to close it out. Depending on the value of your shares, maybe, sell your shares too right after. Retail traders with small balances should stick with their cash balance only. Do not use margin. That way, you do not get a margin call.
Looks like your best option is to get out of part/all of the position as a spread. Enter a spread order to sell 100 shares of stock and buy 1 call. You can see the affect of your total margin requirement when you enter the spread.