Find out what works best for your team. It seems odd that developers/testers would be communicating with customers directly. I would expect that to be the role of the support team who was originally in contact with the customer.
As you said in your comment, they are likely dragging their feet because it's not what they are good at and thus they don't like doing it.
A couple things to try:
- Put everybody in a room and talk it out (doesn't work if there are too many people - maybe just get the dept heads). We need this to get done, it isn't getting done, why not and who has ideas how to fix this?
- I'm not sure why the customers even need a description of what was changed - I'm picturing a "how we fixed it" situation. Who cares how, just that it is fixed. I'm saying to re-examine if this is necessary - perhaps there is an easier suitable substitute.
- Try to automate it. If the customer does need a hand holding explanation of how it was fixed and all they really need to know is that it was fixed, perhaps you could automate your bug tracking tool such that the customer who reported the issue is notified when that ticket is closed - or rather, when it is deployed and visibly fixed for the customer.
Biggest piece of advice is to not make this a blame game situation. Your coworkers aren't unreasonable people - if they are resistant, then perhaps the process is too heavy. Be open to alternative solutions.
FYI - This kind of question may do better over at pm.stackexchange.com