We recently started having an issue where whenever we send a config to a grandstream 2130 (haven’t tested on other phones yet) it won’t store the dial plan as created in the Basefile. The phone only has its default dial plan. Even when you manually put the dial plan in the phone, if it receives another config update from provisioning, it will default.
are the phones local to the pbx or remote? are you provisioning using tftp or ftp?
we recently ran into an issue where we could not provision some phones using tftp. as soon as we switched to ftp they provisioned immediately.
We’re using HTTP for these phones, as they’re newer. Additionally, creating a new template for them to use doesn’t work on the phone either, and when dial plan info is received, it looks like its ASCII equivalent (%20%23%19, etc.). Since we didn’t completely clear the phone when we tested a completely new template, its possible the new template didn’t push any settings (I’d say its likely actually), but we couldn’t risk bringing the site down.
Phones are not local to PBX, and in some cases are behind a firewall, others not behind a firewall. Its not the firewall though, as the proper holes are opened and non-firewall sites have the exact same issue.
We realized that the reason the phone was defaulting was because it somehow ended up in the base file that way, but when we corrected it, it was ignored by the phone - almost like it never regenerated the file. We told it to recreate the file both at the bottom of the base file config screen and at the phone list where you apply the templates. No luck.
have you looked in the config files to verify that the dial plan is correct? have you verified that the phones are actually pulling the correct files? i would also give FTP a try on one or two phones.
you may also have to reset the phone to factory default - we have discovered this the hard way on a number of phones. if we don’t factory reset them they don’t load or worse yet, sometimes seem to only partially load the config. we have seen this happen most often on polycom phones.
if you are using yealink phones, the hardware version and firmware versions can give you headaches as well. we have had to manually update the firmware in some yealink phones, mostly older t26 and t28’s. the newer t4x series seems to be much better.
These are newer phones - they don’t do FTP, and frankly, I’m glad. 
Can cause issues with firewalls. In testing, we’ve taken new phones (cleared) and provisioning still fails. Keep in mind, the original issue was for a phone that had been provisioning for months. I’m tempted to just uninstall and reinstall the module, but I need to make sure i can export/import the existing configs first.