WIFI Calling - Vodafone -fault finding / Application signature ?

Afternoon,

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 88.82.11.221 & 88.82.11.208 

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 ?