It sounds like you're saying you're getting your error message as a voice prompt from the carrier? If so, one thing you might try is to set up ARS rules so you can dial the number at any speed, then when 5000 sees a complete number it grabs a circuit and redials the number at high speed. One way to use this could be to modify your existing ARS, just change the trunk groups in the FACILITY GROUP. If you want to do it manually by pressing 7 instead of 8, make 7 a Station Speed Dial that dials 8-9999 (sends call to ARS, dials 9999, then you dial remaining digits for your call. Add 9999 to the TOLL STRINGS section under Q (Equal access). Now you can write a route group for calls starting with 9999 and send those calls to a new FACILITY GROUP such as "Alt Trunks". As long as that group doesn't include ECHO EQUAL ACCESS in it's dial rules the 9999 will be dropped. Pick your back up trunks as the trunk group for the new FACILITY GROUP, include dial rules ECHO TOLL FIELD, ECHO 3 DIGITS AFTER TOLL FIELD and ECHO LOCAL ADDRESS.
Remember to be sure this rule is above any others the dial pattern might fit into, route group list hunts top-down.
Hint: if you want to use the speed dial button method, did you know that sta spd bin 1 is same as HOUSE PHONE DAY (2 is HOUSE PHONE NIGHT). So if you program a key to be sta spd 1, then type 89999 in the HOUSE PHONE DAY field pressing the key will dial that number. You can then use the copy/paste feature to duplicate for other users.