Thanks for all your recommendations.

I did the change and rr headers only show the fqdn of Kamailio as you can see below in the 200 OK sent back to MS Teams, however still the same issue no ACK.

||||||||||||||||||||
====================
tag: snd
pid: 31232
process: 6
time: 1613047849.768857
date: Thu Feb 11 14:50:49 2021
proto: tls ipv4
srcip: 82.102.42.58
srcport: 5061
dstip: 52.114.132.46
dstport: 5061
~~~~~~~~~~~~~~~~~~~~
SIP/2.0 200 OK
Via: SIP/2.0/TLS 52.114.132.46:5061;branch=z9hG4bK4f5c9b66
Record-Route: <sip:sbc.intelligentsupport.eu:5060;ftag=dcea3c9546914ff8826afca13260da1c;lr=on>
Record-Route: <sip:sbc.intelligentsupport.eu:5061;transport=tls;ftag=dcea3c9546914ff8826afca13260da1c;lr=on>
Record-Route: <sip:sip-du-a-us.pstnhub.microsoft.com:5061;transport=tls;lr>
From: Phillip Kyriacou<sip:+35722010058@sip.pstnhub.microsoft.com:5061;user=phone>;tag=dcea3c9546914ff8826afca13260da1c
To: <sip:+35796552237@sbc.intelligentsupport.eu:5061;user=phone>;tag=as63d965d3
Call-ID: 43f0f0be2fa05b408c81736dbe392aef
CSeq: 1 INVITE
Server: MediaGW V1.0
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO, PUBLISH, MESSAGE
Supported: replaces, timer
Session-Expires: 1800;refresher=uas
Contact: <sip:+35796552237@82.102.42.52:5060>
Content-Type: application/sdp
Require: timer
Content-Length: 339

v=0
o=root 601931926 601931926 IN IP4 82.102.42.52
s=ASTERISK
c=IN IP4 82.102.42.52
t=0 0
m=audio 16360 RTP/SAVP 0 8 101
a=crypto:1 AES_CM_128_HMAC_SHA1_80 inline:IyrJsjVV0w5ORJQ3OJB0r8iyRZU+1lMjg4i9vzC2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=maxptime:150
a=sendrecv
||||||||||||||||||||
====================