We are seeing that Kamailio's nat_helper_module function  of add_rcv_param(),  inserts " (quote)  mark to some of the SIP headers (as below in the contact header in sip 200 OK response)

We got few SIP UAs that are  complaining about the quote mark inclusion in contact header and responding back with SIP 400 bad syntax status code.

Requesting views / suggestions from community about this behavior of kamailio and any workarounds available. 

Cheers

Arslan