(I know this is an old thread, but we 150e users need to build up more info on this list, so here goes)
Well the docs sure don't say much about this.
For certain applications you need the CGM module.
Using the "Assistant T" telephone there is an option 20 - 3 for "reference clock" but the docs don't really say what the options are.
If you are having frequent clock slips there may be some other problem (bi polar violations on one of the T1's?)since the two telco's should be synchronous to each other.