WIFI Calling - Vodafone -fault finding / Application signature ?


Ive got customers using wifi calling behined a XG230

They are complaining of speech dropping out over wifi calls.

fault finging in looking at the wifi of course,

but ref the router diagnosis, I see outbound VPN from the phones on ports 4500 to ip addresses & 

Ive made a outbund rule just to monitor those WAN ip's and call it WIFIcalling


Any better ideas on how to monitor, can i get a application signature for this ?