Extending a pulse from a DTMF Decoder
Do you want the relay to latch on or just stay energized for the length of the tone?

If (a) then just use any of different latches: could use a toggle F/F if you want it to toggle on and off with tones, or capture the rising edge from the decoder and use it to clock an LS74 flip flop.

For (b) you didn't say what problem you ran into the the LS123. I find it hard to believe that it won't configure as a 1-second one-shot, but if that's really the case, you can make very long one-shots using an NE555.

Also, do you really need two levels of isolation from the phone line? You have  both an optocoupler and a relay in your circuit. Better yet, back off and take a 30,000 foot view: what exactly are you trying to build?