Does anyone know how to lengthen the amount of time a user has between dialing digits before the system defaults to "invalid number"?

I tried MMC 501 timers - ISDN Interdigit timer by changing it to 8 seconds from 3 for calls dialed out over the PRIs, but this also added 5 addtional seconds to post dial delay.

Try turning on lcr. That way you can control the interdigit but the call will go out once a match is made in your lcr table.

Is LCR the only way to handle this?

I am having the same issue on a 7200. Never used a PRI before and did not realize this was going to be an issue. The customer is kind of annoyed.

Just the way the Samsung works. Only way to deal with this is setting up LCR. I usually set the ISDN interdigit timer to 5 or 6 seconds, my customers don't even notice. The other option without LCR is to dial a # at the end of the dial string, will send the call immediately.