thanks for the response, checked out that link, also tried the manual provisioning guide, but it did not work,
checked that, ip not blacklisted.Check ip blacklist. Likely blacklisted.
If you factory reset a phone, does it appear as "New" in the Phones tab?
nope haven't seen that "new" under ext. as before I factory reset the phone, I delete the existing provisioning in the extension.
yes very certain that they are in the same LAN. it is a working phone before. i had to reassign it to another user hence i needed to reset it to factory settings. if there are networking issues then i wont be able to provision a t48s in the same network.Hmm that does not sound right to me.. are you sure then that the 3CX machine and the phones are on the same LAN / subnet and can communicate with each other? Sounds like you have some networking issues at the very least that I can speculate.
In this case, your only remaining option is to check the traffic on Wireshark and learn exactly why the phone is not registering.
I will not be able to assist you with that, but you may find this useful:
https://www.3cx.com/3cxacademy/videos/advanced/basic-troubleshooting/
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.