Hi rodneyeid,
Your E90 connect to local LAN or NAT over the Internet? E90 can working fine at LAN. However, it maybe unable to registered to 3CX via NAT over Internet.
Normally, all the SIP clients will work if all the following fields are correct.
User Name: XXX
Password: ***
Authorization User Name: XXX ( same as user name)
SIP Server Name: xxx.xxx.xxx.xxx or sip.xxxxxxx.com
Outbound Sever: xxx.xxx.xxx.xxx or sip.xxxxxxx.com
STUN Server: stun.3cx.com (disable if the phone in use at LAN)
Regards,