Duplicate NAT Source ports in simultaneous calls

[Coolc]

Hi Experts,

I am facing a problem on SIP phones over the public NAT source port on Meraki MX. 

Problem:

Multiple phones are sharing same Public source NAT ports in simultaneously calls to same destination (SBC).

Example:

Phone A makes an outbound call and Meraki MX assign 11786 as source RTP port and SBC assign 20398 for the call and while this is in progress, another SIP phone in same LAN makes a call to same SBC Meraki MX assign the source port (11786) and SBC assign a different port 20478. Due to the dulipcate RTP source ports has been used as above creating problem to with audio.

Question:

How we can ensure that the RTP source is not shared in  simultaneous calls?

Experts your ASAP help would be high appreciated.

Regards

Faz

[PhilipDAth]

The Meraki MX (or any other NAT device for that matter) will not assign the same source port for two active flows at the same time.

[PhilipDAth]

A really good way to improve the reliability of SIP through NAT is to not use UDP.

I personally use SIP over TLS, because it encrypts it, and no form of NAT can mess it up.  SIP over TCP can also help since it allows the NATing router to be able to tell for sure when a call has finished.