Log in to the management interface of your 3CX and do the following:

Go to SIP Trunks

IMPORTANT: You cannot select "Generic" as a country when using 3CX Hosted Interface. You must have 3CX installed on your own hardware (whether it be cloud-based or on local hardware). Here is a link to a forum where a 3CX Staff member states that you cannot use Generic SIP trunk with their 3CX Hosted Interface -> https://www.3cx.com/community/threads/generic-sip-trunk-in-v18-how-to.84233/

• Select ‘Add SIP trunk’
• ‘Select Country’: choose ‘Generic’
• ‘Select Provider’: pick ‘Generic VoIP Provider’
• ‘Main Trunk No’: enter the default number that you want to display, in the international format including a leading + sign. For example + 31202620584
• Click the ‘Ok’ button when ready.

Go to the tab page General

• Enter a name for Trunk. This is for your reference only, so pick a name that will be easily identifiable for you such as AVOXI.
• AVOXI Core: Registrar/Server/Gateway Hostname or IP’: enter "core-sip-qts.avoxi.com"
• AVOXI Genius: SIP Forwarding In Genius IP range 104.196.177.203/32 through 104.196.177.56/32

0ed on your own preference. Click the ‘Ok’ button when ready.

Go to the tab page DIDs:

• Click ‘Add DID’ and enter your DID in international format Without a leading + sign (for example 31202620584) and repeat for all your all other DIDs. Click the ‘Ok’ button when ready.

Go to the tab page Caller ID:

    • At the selection Outbound add 2 replacement rules click ‘Add’

    • Source Pattern 0 and Replace Pattern 31

    • Source Pattern 00 and Replace Pattern +

    • Click the ‘Ok’ button when ready.

Go to the tab page Options:

    • At selection ‘Advanced’, select ‘Support Re-invite’, and disable ‘Force invites to be sent to IP of Registar’

    • At selection ‘Codec Priority’, only allow codec G729

    • Click the ‘Ok’ button when ready.

Go to the tab page Outbound Parameters:

By default the Caller ID from Extension settings are used in case of Call forwarding to outbound calls; however, in case you want to see the original caller ID of the dialler with ‘Call Forwarding’, you need to change the following details:

• Change Display Name to ‘Originator Caller number will be sent’
• Change User Part change to ‘Originator Caller number will be sent’
• Click the ‘Ok’ button after making these changes.