The CAD (Computer Aided Dispatch) system takes care of that by sending the closest trucks regardless of whose "area" they're in or whose "area" the call's in. That's something that's taken a long time for the fire crews to get used to, as they all have "inspection districts" which they're responsible for. They think that they will only get calls for that area, and that only they will get calls for that area, but that's not the case.
The computer links with GPS in every front-line truck to know exactly where any truck is at any given time. (If the GPS fails, or in trucks without GPS, the computer 'falls back' to a system that relies on the last known address for a truck (i.e. the call it was dispatched to, for example, if it has cleared the call and is available on radio).) When the dispatcher calls for a "recommend", he/she's asking the computer to find the closest x trucks of type y for a call at z. So for example, if there was a possible building fire at the postal plant, at 1100 49 Av NE (behind the Tim's/Wendy's on McKnight), when the dispatcher hits "recommend", the computer is instructed to find the two closest PUMP, the closest AERIAL, and the closest EMERG. If everyone's in their halls (normally), this would be 27P 18P 18A 4E. However, 18A is down right now for maintenance, and 27P is in 13 station. So the recommend would be, most likely, 18P 27P 12A 4E.
But wait.. what if 10 Squad is coming off Deerfoot x McKnight and on their way to 32? They're still available any time they're not on a call. So because they're where they are, the recommend will probably be 10SQ 18P 12A 4E, and 27P stays "at home" in 13.
Now let's say 31E is just clearing a call at Deerfoot x 64th southbound. 10SQ 18P 12A 31E.
It's a very dynamic system, and it's why you hear some crazy recommends sometimes. When the GPS were first being put in fire trucks, I once had 32 Pump on Blackfoot Trail having just left the shop at 16. A call at Alyth Yards sent 32P 16P 1A 4E. Had to explain it to the captain on 32 afterwards.
All that said, the answer to your question is, part of it will be covered by 18, part by 4, part by 7, and part by 27. The computer auto-adjusts to take 27's new location into consideration.