ooh..that's the kicker...we get some of the same questions only we work it in reverse (the long distance dms-250 usually is set up to match the customer switch)...
If I've read the posts right, your options in the Definity are DMS-100 custom, DMS-250 and NI2, correct? If it's a local PRI, then it's probably the DMS-100. I'm not clear on the options in that switch. But using the DMS-100 custom option probably requires that there's a setting on the local carrier switch that requires using SL1 profile. Normally, that's only for other NorTel meridian pbxs, but in our experience, using the SL1 profile on the LEC/IXC switch loaded against the NorTelNorthAmericanPRI (NTNAPRI) protocol standard works better.
Q. What are the troubles with the d-channel? Is it not coming up at all or is it dropping out? There may be an issue with the way the d-channels sync up. The LEC/IXC switches sometimes run SPM vice DTCI cards (difference in hardware, but can play a huge factor on the d-channel). If the T-1 is running to an SPM, you may have to check to see if there's a CRC6 enable option on your end -- the D-channels on the SPMs sync over the CRC-6 bits; if you don't have that enabled, the d-channel will eventually come up but it could take anywhere from a couple minutes to a couple hours. We've also seen in Definity switches where the customers are using inverse polarity on the d-channel, which stops the d-channel from coming up at all.
I don't know who's switching your traffic, but you should be able to use the Lucent Protocols just fine (the LEC/IXC will probably have to use the U.459/5ESS protocol standard). Personally, since our network is all DMS-250s, I try to talk the customers into the NorTel protocols - but a significant number of our customers use U.449 and U.459...their call.
Are you using NFAS? We've seen troubles where the customer equipment was ordered using a back-up d-channel but the cpe was not capable of supporting a backup. We've also seen orders where the customer requested two T-1s on one trunk group, but one active d-channel per T-1. That's prohibited in the trunk tables in the NorTel switches...the symptom is usually the LEC/IXC sees one d-channel come up, the other stays in lockout and you'll swap them back and forth and it won't change...the customer usually sees them both in service momentarily but they don't stay that way.
Hope it helps a little bit. I know only a little about the Definity side, and I know the IXCs are severely restricted in the options they can provide (without extra cost).