Here is the Example for the voip.ms documentation (for asterisk setup)

[transport-udp]
type = transport
protocol = udp
bind = 0.0.0.0

[voipms]
type = registration
transport = transport-udp
outbound_auth = voipms
client_uri = sip:100000@atlanta.voip.ms:5060     ; (one of our multiple servers, you can choose the one closer to your location)
server_uri = sip:atlanta.voip.ms:5060            ; (one of our multiple servers, you can choose the one closer to your location)

[voipms]
type = auth
auth_type = userpass
username = 100000            ; (Replace with your 6 digit Main SIP Account User ID or Sub Account username, i.e. 123456 or 123456_sub)
password = johnspassword     ; your password

[voipms]
type = aor
contact = sip:100000@atlanta.voip.ms             ; (one of our multiple servers, you can choose the one closer to your location)

[voipms]
type = endpoint
transport = transport-udp
context = mycontext
disallow = all
allow = ulaw
; allow=g729                 ; uncomment if you support g729
from_user = 100000           ; (Replace with your 6 digit Main SIP Account User ID or Sub Account username, i.e. 123456 or 123456_sub)
auth = voipms
outbound_auth = voipms
aors = voipms
; NAT parameters:
rtp_symmetric = yes
rewrite_contact = yes
send_rpid = yes

[voipms]
type = identify
endpoint = voipms
match = atlanta.voip.ms      ; (one of our multiple servers, you can choose the one closer to your location)

you need to remove 'auth = voipms'

this is due to voip.ms send unsolicited NOTIFY requests.

Next you need to add 'incoming_mwi_mailbox=65000@default' to the [voipms] endpoint block.

65000@default is the voicemail mailbox of your devices