Question : How to challenge ISP saying our T1 is good when its not

I am at a loss on how to resolve an issue with Verizon Business.  We have a burstable T1.  I am told that the term burstable only means the pricing model that is used and we should have a full 1.54 of bandwidth.  When we run speed tests, the bandwidth varies from 400K to 1M.  A trouble ticket with Verizon tells us that the circuit is fine, there is no packet loss, and the pings are coming back at 60ms, which means it is fine.

What can I do to pursue this?

Answer : How to challenge ISP saying our T1 is good when its not

The term "burstable" is just a billing term. You contract for, say 256k w/Burst to 1.5M
Verizon will monitor your line utilization throughout the month and if your average is over the 256 then the next month you may have to pay for the next higher billing increment. Drop back to below 256 and your billing rate goes back down. Good news is that they take samples periodically 24 hrs/day meaning that many of their samples will be taken nights and weekends that will be averaged in with your normal workday traffic.

I've never heard of any situation where a T1 is part of any "shared trunk" unless it is frame-relay. Frame-relay uses statistical multiplexing where a single T1 can service many clients. If your drop is frame-relay then you will also have a Committed Information Rate (CIR) which is used to manage the bandwidth.

I would be more interesed in the error counters on the router interfaces. Any CRC or frame errors point to local wiring, static on the line or "bad pairs" under the street somewhere, or a DAX going bad, or the smartjack, or timing issues between the CLEC and the ILEC. All of these will affect performance.