Hello,
Thank you for your message. Let me summarize your situation for more clearly:
1. You have asterisk PBX ( with FreePBX ) on location 1. There is 1 IP phone ( with 3 SIP accounts). This phone can register to PBX and making call ( incoming calls & outgoing calls) properly. NO problem with this site
2. There are 5 IP phones : 3 Cisco + 1 Linksys + 1 Gigaset (with 10 SIP accounts) on location 2. Cisco & Linksys phone cannot register to PBX at Location 1. Gigaset can register to PBX at location 1 but no sound when making call.
3. There is 1 IP Phone: Gigaset ( with 1 SIP account ) at location 3,4,5. They can register to PBX at location 1 but there is no sound.
In my opinion, your trouble is networking trouble. All you need to do is fixing network issue with NAT.
As you may know, you must open these port on Modem (Router) as following:
SIP trunking:
1.1 Open SIP port 5060 - UDP (for SIP signal )
1.2 Open range port from 10000 to 20000 - UDP ( for audio )
On FreePBX, configure SIP extension using NAT
In case you still get trouble, you could try with IAX. IAX only need open 4569 port for all.
I think I can fix this trouble for you. If you still have any question, please let me know.
I rebid and hope we will work together ASAP.
Rgds,
Vu Le